Before we get carried away about how overpowered and game breaking this feat is.... lets review some of the "facts" presented.
Our party used the following. Bard would cast bless. Action Surging battlemaster would sit on his Battlemaster Dice to use precision for those attacks he missed.
Let's say attack sequences looks something like this.
1. Wizard casts fly on GWM fighter.
2. Fighter flies in to dragon. Uses Action Surge. He has a +1 sword and is 11th level. Adult Dragon AC is 18. He gets +4 prof. +4 str. +1 sword +1d4 with bless.
Hit roll: +11 without GWM Dam: 2d6+5 for 12 average damage.
Hit roll: +6 with GWM. Dam: 2d6+15 for 22 average damage
Against an AC of 18.
Average percentage to hit without GWM: 65%
Average percentage to hit with GWM: 40%
So average damage against AC 18 without GWM with 3 attacks per round equaling two hits: 24 points
So average damage against AC 18 with GWM with 3 attacks per round equaling 1 hit: 22 points
Okay, a few problems already.
1) You skipped GWF, which changes the damage to 13.33 and 23.33
1a) You didn't account for damage from crits
2) You miscalculated the percentages, should be 70% and 45%
3) You did some pretty arbitrary estimating there....
Taking GWF into account, using the correct hit percentage, and using actual calculations instead of estimations.... we get
Damage per round from 3 attacks (no GWM): 29.2
Damage per round from 3 attacks using GWM: 32.7
So far GWM is not even close to being a problem. (A whopping 12% boost)
It should also be noted, that next level, when Str bumps to 20, the benefit from GWM gets *smaller*
Now incorporate the nova attack blowing Battlemaster Dice adding 1d8 to each attack roll that misses and having a bardic inspiration dice as well, an average bonus to hit 4.5.
Damage without GWM: 36 points
Damage with GWM three hits: 66 points.
Now is when everything falls apart.... Somehow you decided that using Precision Attack meant that *every GWM* attack hit...??? Wut?? You somehow jumped from 22pts to 66 pts....??
Lets try using real numbers and see what actually happens.
Lets be really nice and assume a +5 from Precision Attack (more on this later). Even with that the damage goes to...
3 attacks without GWM: 39.3
3 attacks with GWM: 50.3
So your "80%" turns out to be more like 28%. And that assumes a +5 from Precision Attack.... which brings us to the second problem.
Precision attack only gives a 4.5 bonus if you use it *every* time you miss by 8 or less. And no one does that. In reality, people tend to save them for more reliable results. So if they miss by 3-4 or less the spend the die, not when they miss by 7. Otherwise you tend to 'waste' more of the dice and they are a limited resource.
For someone playing it safer, and only using it if miss by 3, that comes out to a 2.6 bonus, (we will still use +3 because of you rounding down Bless). Damage now becomes"
3 attacks without GWM: 35.3
3 attacks with GWM: 43.3
Now that damage bonus is just under 23% (for completeness sake.... if using Precision on a 5 or less, the damage bonus is just over 25%)
So here we have close to a 'worst case scenario'. An 11th level fighter, with precision strike, with a bard casting bless, with a magic weapon....and we get a boost of about 25%.... for as long as the Battle master dice hold out.
More importantly, that means his Nova attack, with Action Surge, does an extra 20ish pts of damage on round 1. Which against that Adult Dragon is a whopping 10% of total Hit points.
So.... why is this such a horribly overpowered feat....???
The smiting paladin is more theory-crafting since he did not have GWM. A 3rd level spell with greater divine smite does 5d8 smite damage. With longsword that is 2d6+5 plus 5d8 for an average 39 point hit. GWM adds +10 damage. It's around a possible 26% increase in damage for a smiting paladin. Not quite as bad as the fighter. Given the paladin has two vows that completely eliminate the -5 penalty, he probably hits more often than the fighter in single target situations.
You seem to keep ignoring that the more damage a fighter can do, the worse GWM performs, it means that -5 has a larger negative effect. There is no such thing as 'eliminating' the -5 penalty, until the AC is 3 lower than the attack bonus, it will still have an effect.
Lets say the Paladin is willing to Smite with a 3rd level spell on every hit. Assuming the same stats as the fighter, but no magical weapon, we have a +10 attack, so needs an 8 to hit. Lets assume a Vengeance using his Vow, so with Advantage
So the Paly is doing almost 35pts per hit,
2 attacks without GWM: 67.1 hp
2 attacks With GWM: 63.4 hp
Yes, the Paladin will do *LESS* damage on average by using the -5/+10 trade off. So no, not a "26% boost", more like a 7% Drop.
So, again, I have to ask.... what is so scary about this feat...??