2) The mathematical average disregards the fact you can CHOOSE not to use -5/+10. Please take this into account by assuming the GWM fighter to use +0/+0 above the AC breaking point (say AC 14 theoretical unbuffed, AC 18 practical play).
I would like to point out that unbuffed, the break even point is about AC 14 at level 4 (or level 1 for a human variant GWM PC).
At level 5, it's AC 15.
At level 9, it's AC 16.
At level 13, it's AC 17.
At level 17, it's AC 18.
It increases by 1 every 4 levels without a single buff, a single debuff (or condition), or a single bonus to hit magic weapon.
However, as CR increases, AC also tends to increase. The question is, how much?
Theoretically according to the DMG and MM XP/CR guidelines, a CR 1 creature is about a medium encounter for a party of level 1 PCs and a CR 20 creature is about a medium encounter for a party of level 20 PCs.
Just looking at the CR 4 and higher demons and devils out of the MM (admittedly, a small sample size), monster CRs, monster ACs, the GWM break even point, and the delta between monster AC and GWM break even point are:
Code:
4 13 14 -1
5 15 15 0
5 15 15 0
6 15 15 0
6 15 15 0
8 16 15 1
8 16 15 1
9 17 16 1
9 19 16 3
10 15 16 -1
11 18 16 2
12 18 16 2
13 18 17 1
14 18 17 1
16 18 17 1
17 19 18 1
19 19 18 1
20 19 18 1
As a rough rule of thumb here, same CR monsters tend to have ACs 1 higher than the break even point (median for demons/devils is 1 higher, average is 0.78 higher, but it typically ranges from 1 lower to 2 higher).
Upping this to a deadly encounter means multiplying the XP by 2, an AC increase of 0 if we do this by having 2 foes, or by about 1 for a single foe. The CR increase is 1 at low level and 3 at high level. But, upping the CR by 3 does not typically mean an AC increase of 3, usually only an AC increase of 1.
So for a medium encounter where the party might not be buffing too much, the AC of a single creature is about 1 higher than the break even point.
For a deadly encounter where the party will often be buffing, the AC of a single creature is about 2 higher than the break even point and the AC of two creatures is about 1 higher than the break even point.
Using more lower CR creatures usually (but not always) means ACs at or below the break even point.
So it seems to me that it's not too hard to get a +2 to +3 increase in to hit (to go above the break even point) when facing a deadly encounter. A +1 weapon alone is +1 of that, a +3 weapon is +3 (and let's admit that most DMs do eventually hand out magic weapons with a bonus to hit). Bless is +2.5. Advantage (prone, blind, faerie fire, held, feinting, restrained, etc.) against a higher AC foe is +4.5. And finally, most melee PCs will eventually increase their Str (or Dex) to 20 and that is +2 more at higher levels.
Not only does GWM do more DPR straight out of the box than +1/+1, but it eventually leverages to more against the types of similar or higher level challenging foes most PCs encounter at most tables.
Whether this is broken or not is dependent on how much the DM hands out magic weapons, how much the players optimize their PCs, how many different ways the party has to increase to hit with buffs, spells, and conditions, how much players of other melee PCs are bothered that the GWM PC is doing 2 times the damage their PC is doing (it's not just the +10, it's also bonus attacks for a foe dropping to zero or a critical being rolled), and how much the DM gets annoyed when his carefully planned out BBEG is taken out in 2 or 3 rounds.
