Will actually do more damage with full build than old Q. Here is the math, it's actually entirely possible for him to even have more AD than these calculations were done on. It's a early game nerf and extreme (30-35min) late game buff.
So let's say with a build that gives him 400 AD.
*Anything over his base level 18 damage is Bonus AD including runes and masteries* * Base AD at 18 is 105*
New Draven Q: 50 flat damage at max rank + 105% Bonus AD, so 400-105 = 295. 105% of 295 = 309.75
309.75 + 50 = 359.75.
Old Draven Q: 85% of Total AD so 85% of 400 is 340.