That's a about right for the Minolta, as the Jaguar XJR9 IRL got to 250+ and it's from the same class as the Minolta. Those Group C cars were actually dangerously fast, anyone remember Stephan Bellof in the Porsche 962c, he got that thing cooking on the Nurburgring. Not like those cars aren't capable of going that fast especially in a slip stream. Real life you have to worry about the car shaking and a myriad of things going wrong that can end your life. GT5 should have added the element of directional wind, and wind gusts. Make things a little more interesting, perhaps.
Nascar is a whole different story, if you don't have someone to bump draft, hell you are screwed on high speed tracks. That Indianapolis race, you needed to out brake them to win, slip stream wasn't enough, because the other cars were bump drafting. Bump draft and slipstream out of a corner to a bump draft then pass by braking slightly later and then block, because the AI line is obviously faster than what you took by late braking.
Can't say I've noticed anything weird on the online side, as people tend to either had really powerful cars already or I was faster coming off a corner. Will look for more, but the slip stream isn't overpowered at all.
I'm wondering will it be possible for some cars to be flipped because of a combination of speed, rough surface and suspension settings?