Saying everything is the same after 80fps is just retarded, who's the authority on that? Who's studied the game and decided this must be true? It's a load of bollocks, try cabbie boosting with 80fps vs 140fps, there's a big difference. It's exactly the same with bikes, the difference from 40 to 80 is the same as 80 to 120 and so on. It's all just relative to the difference having more FPS makes to a particular vehicle.
When it comes to bikes your RAD wont get much smoother past 80fps, but there's still a minor difference and you'll be able to go faster because of that. The main differences you'll notice on a bike after 80fps are that it becomes easier to wheeliee and lean forward (which will help you keep your speed during the runup to a stunt). It also becomes easier to stoppie, easier to land and control grinds, and easier to maintain your speed easier in the air (which gives you more height and distance in return).
I was going to write something but just quoted this since it's what I feel. Just upgraded my drivers and my FPS went from 80-90 to 130-150 just as I had them before, noticing the stuff SS wrote.