I played it with six other AI cars, smooth as silk. Now, whether more cars would result in a slowdown, I can't say cuz I haven't tried it yet.
Then I would suggest actually doing what I said, rather than assuming I'm wrong.
Put 10 or more cars on the grid (like a real race, for God's sake), and see what you get: massive stuttering. This does not occur on Nvidia hardware.
Also, you're glossing over the fact that, in Career mode, all the races have over 10 CPU cars - thus rendering those races only marginally playable. This isn't some tiny little problem - it pretty much breaks the game. Unless, of course, you don't care about the single player career at all (in which case you will never get to drive 80% of the available cars since you'll never level up) , and never race with more than 6 other people.
Sorry - no way is that "smooth as silk". And my system is the same as yours, just with a better CPU. I seriously doubt you'll be getting better performance. So Yes - I think I can intelligently extrapolate that your system will choke just as badly under the same circumstances as mine. If you claim otherwise, I'd like to hear why.