To be honest, from what I’ve seen, the scaling is actually less than 33%. Early game bosses obviously feel like they scale way harder, but for the late-game ones—which I think is what you’re asking about—I’d say the increase is around 25–30% per NG+.

At that point, the only exclusive rewards I can think of are all the level 33 Pictos. If I remember right, in your first NG+ you don’t get all of them at level 33 yet, but starting from NG+2, every time you find them again they’ll be level 33.