It seems to me the fix would be to change the rarity of power-improving mechanics. If it takes 100h to get that 10% improvement (say, a yellow item that gives one more mod), then 5% of the population will have enough play time to get it and you have that problem. If the rarity is changed, say to 10h of play, a large percentage of the population can get it, and problem fixed. You balance for them.

Problem is, of course, that very active players get bored since they can reach perfection pretty quickly, relatively speaking. You could keep them happy with more content, but that's probably not an option.

My suggestion would be to reduce the number of mods, say to 4 top instead of the 6/7 you can get today (depending on slots), and make that goal reachable for most players. Then add these rare drops with a nice color on them (yellow!) that don't give one more mod, but a fixed boost. Make it 1% universal damage on main hand, and 1% to all healing on legs, whatever.

Very active players can still hope for a set better than average, but don't throw away the balance too much.

Being an extremely active player myself, I realize this would make the game less interesting, however you could compensate that by giving extra perks to these rare stuff. For example, the ability to dye them, or something different but cosmetic only. Or +1 inventory slot. Possibilities!