Right I had a play with this and the GT3 Cup from Porsche. I don't think this will work. I had a go around Hockenheim and, although the lap times were comparable, they are two completely different beasts. I ran Wayne's RUF and the stock, untuned GT3 Cup.
The RUF is heavy and floppy with hugemongous amounts of engine grunt whereas the Cup is tight and pointy, and can stop on a dime. If you raced with these cars you'd end up with two groups basically and tracks would favour one over the other. Hockenheim is a good tester as it's got straights and corners, but you get a twisty, technical track and the Cup is going to piss all over the RUF, whereas if you get a track with lots of long straights, the RUF is going to blunk in the Cups' faces.