Sylas W heal as of right now is by far the biggest complaint people have about him and for good reason. Maxing it first in lane makes him ridiculously powerful in duel, to the point where at lvl 5 he outscales other duelists in their own game.
With that said, I do not believe he needs an outright nerf either. Just a shift in power to make the skill fairer to deal with. So my idea would be to shift the growth to be based on his level rather than the skill level. Like that, he doesn't have the literally biggest heal in the game by lvl 5 (Not even overexaggerating sadly)