2 points of Effective Damage per Attack is a big deal though. A very big deal. When most characters deal less than 10 Effective damage per attack then +2 to that is a big deal. Could be 20-40% of a difference in damage.
I think I missed the point of this assessment. But taking the change in AC to the extreme in case it is useful...
Having the target go up 2 AC means a loss of 1 effective damage per attack for non GWM and a loss of 2 for the GWM fighter or a difference of an additional -1 for the GWM fighter. At AC 19, the regular fighter does about 1 DPR more than the GWM fighter at level 1 but is still 10 DPR less at level 20 because the GWM fighter gets better chances to activate the bonus attack with each additional attack.
Essentially, the only time a GWM fighter should not use power attack is at level 1-4 against high AC targets and when they have disadvantage on the attack roll.
Even then, a DM either gets heartburn from GWM or they do not. I have not had an issue with this or sharpshooter. Enemies die faster. Yes, and? The GWM fighter that was in our level 1-20 adventure path was valued by the other players rather a point of annoyance. They worked together to provide optimal situations for the fighter, but they sacrificed resources to do so, and the damage output saved the PCs on more than one occasion.
A DM has 4 main options I can think of if they find GWM to be breaking things:
* Treat their encounter building as if there is 1 more PC, especially against significantly weaker enemies
* Remove GWM from the game.
* Choose the GWM power attack modification from page 3 of this thread that allows extra damage while giving them the least heartburn.
* Turn GWM into a half feat that gives +1 strength and the bonus attack Cleave option.
And then they move on.