Just to recap the DPR effects of the -5/+10 mechanic, here is a table. These calculations use a d10 weapon (avg 5.5 dpr), +4 proficiency bonus, +6 attack bonus (+4 main stat, +2 magic weapon or other buff) against a 17 AC target, for a character having one attack or four attacks. This is a feat, chosen against a +2 ASI.
[TABLE="width: 676"]
[TR]
[TD="width: 64, bgcolor: transparent"][/TD]
[TD="width: 64, bgcolor: transparent"]
Hit Chance
[/TD]
[TD="width: 64, bgcolor: transparent"]
Dmg/Hit
[/TD]
[TD="width: 64, bgcolor: transparent"]
1 Attack
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/Adv
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/ Disadv
[/TD]
[TD="width: 64, bgcolor: transparent"]
4 Attacks
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/Adv
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/ Disadv
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
Before ASI
[/TD]
[TD="bgcolor: transparent, align: right"]
0.7
[/TD]
[TD="bgcolor: transparent, align: right"]
9.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
6.7
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
8.6
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
4.7
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
26.6
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
34.6
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
18.6
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
+2 ASI
[/TD]
[TD="bgcolor: transparent, align: right"]
0.75
[/TD]
[TD="bgcolor: transparent, align: right"]
10.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
7.9
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
9.8
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
5.9
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
31.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
39.4
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
23.6
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
-5/+10
[/TD]
[TD="bgcolor: transparent, align: right"]
0.45
[/TD]
[TD="bgcolor: transparent, align: right"]
19.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
8.8
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
13.6
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
3.9
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
35.1
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
54.4
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
15.8
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"][/TD]
[TD="bgcolor: transparent"][/TD]
[TD="bgcolor: transparent"][/TD]
[TD="class: xl65, bgcolor: transparent, colspan: 3"]
-5/+10 on first attack only:
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
28.7
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
39.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
17.9
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
+4 ASI
[/TD]
[TD="bgcolor: transparent, align: right"]
0.8
[/TD]
[TD="bgcolor: transparent, align: right"]
11.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
9.2
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
11.0
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
7.4
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
36.8
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
44.2
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
29.4
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
+8 ASI
[/TD]
[TD="bgcolor: transparent, align: right"]
0.9
[/TD]
[TD="bgcolor: transparent, align: right"]
13.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
12.2
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
13.4
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
10.9
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
48.6
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
53.5
[/TD]
[TD="class: xl66, bgcolor: transparent, align: right"]
43.7
[/TD]
[/TR]
[/TABLE]
We see that for normal attacks, the -5/+10 mechanic is equivalent to a
+4 ASI.
For attacks with advantage, the -5/+10 mechanic is equivalent to a
+8 ASI!
Note that disadvantage is much less common, and a -5/+10 character can opt not to use that feature in that case.
Dropping to once per turn makes it much closer: a big bump for single attack characters, but not so good for four-attack characters.
(Remember that the feats in question and +2 ASI all have additional, non-attack/damage benefits. For simplicity, let's assume these roughly balance.)
If you prefer the always-on mechanic, I suggest changing -5/+10 to either -2/+3 or -3/+4:
[TABLE="width: 676"]
[TR]
[TD="width: 64, bgcolor: transparent"][/TD]
[TD="width: 64, bgcolor: transparent"]
Hit Chance
[/TD]
[TD="width: 64, bgcolor: transparent"]
Dmg/Hit
[/TD]
[TD="width: 64, bgcolor: transparent"]
1 Attack
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/Adv
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/ Disadv
[/TD]
[TD="width: 64, bgcolor: transparent"]
4 Attacks
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/Adv
[/TD]
[TD="width: 64, bgcolor: transparent"]
w/ Disadv
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
+2 ASI
[/TD]
[TD="bgcolor: transparent, align: right"]
0.75
[/TD]
[TD="bgcolor: transparent, align: right"]
10.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
7.9
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
9.8
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
5.9
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
31.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
39.4
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
23.6
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
-2/+3
[/TD]
[TD="bgcolor: transparent, align: right"]
0.6
[/TD]
[TD="bgcolor: transparent, align: right"]
12.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
7.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
10.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
4.7
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
30.0
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
42.0
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
18.6
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
-3/+4
[/TD]
[TD="bgcolor: transparent, align: right"]
0.55
[/TD]
[TD="bgcolor: transparent, align: right"]
13.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
7.4
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
10.8
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
4.7
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
29.7
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
43.1
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
18.6
[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]
-3/+5
[/TD]
[TD="bgcolor: transparent, align: right"]
0.55
[/TD]
[TD="bgcolor: transparent, align: right"]
14.5
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
8.0
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
11.6
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
4.7
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
31.9
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
46.3
[/TD]
[TD="class: xl65, bgcolor: transparent, align: right"]
18.6
[/TD]
[/TR]
[/TABLE]
These are not as extremely powerful with advantage but are still solidly better than a stock +2 ASI, but they create the need to choose wisely, as without advantage they are better than no ASI at all, but not as good as a +2 ASI. (Disadvantage numbers come from the no-ASI numbers, which are better.)
Going -3/+5 would be strictly better than a +2 ASI even without advantage, but not as overpowered as -5/+10.
In summary:
Regardless of the desire for a melee fighter to be the best damage dealer of all, a feat should match a +2 ASI. Short of rewriting the feat entirely there are two solutions:
change both GWM and SS to once per turn, or change them to -2/+3 or -3/+4.
Addendum:
Crossbow Expert can be modified, too, such as dropping the disadvantage-cancellation in melee to once per turn (helping only single-attackers). This will make the crazy combos much less potent. You could also specify that the bonus action crossbow must be a different crossbow.
Crossbow Expert
Thanks to extensive practice with the crossbow, you gain the following benefits:
- You ignore the loading quality of crossbows with which you are proficient.
- Once per turn, being within 5 feet of a hostile creature doesn’t impose disadvantage on your ranged attack rolls.
- When you use the Attack action and attack with a one handed weapon, you can use a bonus action to attack with a hand crossbow you are holding in your other hand.