I tried out a stock Audi R8 LMS Base model, and man was it not what I was expecting!
I found that the feeling of driving a race car is so much different than GT5. I definitely think tire heat is much more refined, because when I was doing some arcade racing, the first 1-2 laps (on RH) I was spinning out and sliding when I didn't think I would. But once I got a lap or two down, I actually saw less spontaneous loss of grip.
Been really wondering about this for a while. Another issue, that I believe has been already pointed out, is that MR cars seem to like spinning out like crazy (maybe not all but quite a lot of them). I was using something like a Toyota MR2 and it was way too easy to spin out!