Hmm. Fair. I haven't seen the GWF reroll slow things down too much, but that doesn't mean it can't.
I still think the solution may be to find a way to up the average damage, though, rather than the maximum. Or to up the max a little (flat +2?), and add some other benefit in addition.
I do not think, with respect to those who've suggested it, that a 5/10 feat for other styles is the way to go. I get wanting the different styles to be equal, but they shouldn't be identical. Dueling, finesse, all of that? Should offer a different sort of bonus than GWF.
Perhaps--again, top of my head, haven't analyzed it--a duelist-related feat, rather than providing a 1/round defensive bonus that takes your reaction, could instead apply a defensive bonus for an entire round against a target that you successfully hit and damage with a one-handed weapon?
Well, if we're going to analyze things and make them "more balanced" then we should take a longer look at them.
We have three to six basic melee styles we're looking at, plus 2 ranged, and one hybrid. There's also two weapon-mage styles, but that's an unnecessary tangent.
Great Weapon (/Polearm)
Two Weapon Fighting
(STR/DEX) sword/board +(whip?)
Longbow and Shortbow
Rapid shooting Crossbows
Sword and ranged weapon combo
Whip and the Sword/Ranged don't have any Weapon Mastery feat support (or are maneuver), so lets set those aside for now. Most of the mega weapon style feats in 5e are actually old 3e style feat chains. Great Weapon Mastery is a combination of Power Attack and a few other things. Shield Mastery includes shield bash, shield evasion, etc. Duelist is kind of like the Combat Expertise feat. Dual Wielding combines Two Weapon Defense, Quick Draw, and that one feat that let you dual wield larger weapons.
Now, keeping in mind that a lot of these old feats ended up as battle maneuvers (trip, disarm, feint, etc) or just part of the system, we can look at other feats and try to boost up the power of these weaker "weapon style" mega feats.
To boost TWF, we can see about adding in more offhand attacks (as the TWF chain of 3e did) - part of the whole duel wielding is the large amount of off hand attacks in a flurry of blows. Maybe at a penalty. Boost that defense bonus up to +2; it should be equal to a heavy shield (again, like the 3e TWF chain).
That's how I would do it. Look back at 3/4e, find feat chains, and mimic the styles that way. Tweak numbers until balanced with GWF and Sharpshooter.