You still have to hit your opponent's full AC on one of your attack rolls.
Now you're talking about "Fell", right?
At the moment it sounds a smidgen clumsy, the whole idea to check both ad/disad results.
If we can end up with something simpler - something that works just the same regardless of advantage or disadvantage or "no vantage" - that'd be swell.
In this case: "if you hit AC 16 you deal [Prof] even on a miss".
Of course, if [Prof] bonus damage were okay:ish, we could simply the whole feat to:
GWM
You've learned to trade accuracy for momentum.
If you hit AC 16 on a melee attack you make with a weapon you are wielding in two hands, you deal your proficiency damage in addition to the damage you deal on a hit.
Example: You are level 12 (proficiency bonus +4) and attack an Umber Hulk (AC 18). Your attack roll results in 17. Since you reach 16 or higher, you deal 4 damage even though you missed. Your next attack roll is 20, a hit. You deal 4 damage in addition to your regular damage.
But I guess we're back in "too good" territory now... Besides, coming full circle makes the feat even simpler.
Like so:
GWM
When you wield a melee weapon in both hands, if the attack hits, you add your proficiency bonus to the attack's damage.
One thing is clear - this feat is
straightforward!