I've been doing the 'ring with 500pp cars quite a bit recently and I've found the Lexus LFA can do it in under seven minutes. Toyota FT86 Gsport is almost as quick. I've seen plenty of people online do around the 7 minute mark in NSX type R's, RX8's and Maserati Gran Turismos. All just a matter of preference really, I'm sure you can go fast in a large amount of cars. I would say avoid 4wd's, they seem to lose out on power/weight at 500pp.