Well, I see this took off a lot more than I thought it would LOL. Gone for a 12-hour shift and dozens of post to read.
I can easily see both sides of the debate. I can also see how some players and DMs might feel GWM and SS are too powerful. I've looked at the numbers briefly, and in retrospect I don't feel a -X/+2X base rule would not be a good idea. First off, if it is a base rule, it takes away from the feats. Second, because the concept is variable (any penalty to attack for twice the bonus to damage), it is subject to even greater abuse than GWM and SS are at present according to some points of view. Third, players who favor casters might argue for it to apply to spell attacks as well.
From a mechanical perspective, a -X/+X is much more reasonable. Limiting GWM and SS to once per round also helps (that has become our group's house-rule). But my players also explore their character types and we have a pretty decent variety:
Tiefling Barbarian with GWM and a Greatsword (now uses a Greatspear, similar to a Lance in 1d12 damage).
Wood Elf Ranger with SS and longbow
Dragonborn Fighter (Knight) with sword and shield in Plate +1 (tank-type)
Half-Orc Rogue (Assassin) dual-wielding Shortsword and Handaxe (dipped into Fighter for two weapon fighting style).
Other PC "warriors" currently absent:
Drow Elf Ranger (archer style) but no SS, also dual-wields but no style or feat taken yet.
Duergar Fighter (tank) with Plate/Shield and War Pick. Likes to attack with shield as off-hand weapon so took two weapon fighting style).
Then there are the clerical and other types, using weapons secondary and spells, including cantrips, more often.
My point is simply that for our group thus far, no one has been awed by the use of GWM or SS, and I don't see it being much of an issue in the future. It has popped up a couple times and been useful (against low AC foes, such as Zombies at AC 8), but the player who uses them (he plays both of those characters) is often just as disappointed he missed because of the -5 penalty as he is happy when it works.
Anyway, good luck to all in whatever path you choose. Cheers.