Well I just tried the TT and ran a 3:30 with the '01 R8 - not very fast I know, it was a pretty wobbly run. But that was my fastest car so far, I tried the 787B and the Bentley but could only get down to around 3:35-7 with those.
The only tuning I'm doing is setting the LSD to around 7/17/7, dropping the suspension to minimum height +5, power limiting to meet 700pp, and transmission top speed of course (about 255mph)