Sounds like fun! But you should set a standard with what aids are being used and the such. Cos a driver using full aids will never go off the track, or slide, or whatever, and a guys shifting manually may get better times, so there has to be some sort of balance. The only aid I use in Forza 4 is the cornering line, so I am sure my time would be different from someone using the full racing line etc.
Another cool thing is to do a top end test on the old Le Mans track where there are no chicanes down the Mulsanne straight.... Oh yeah! Got a Veyron at about 420km/h down that straight!