With CFW PS3, anything is possible. If you want to know more, visit dedicated forum that deals with CFW PS3, there would be people there who play GT6. Even in seasonals, regularly cheaters appear on the leaderboard top rank. I also think even with cars given by PD during TT, these people can still do 'stuff', maybe just alter the tune ( suspension, lsd, aero ) to suit their driving
These small changes can help a lot as stock tune usually used by PD is suck, and it would not be easily noticeable just by watching the replay.
The only people who would do this on GTA TT to get on the finals, would be those who confident with their real life car driving skills and experience, but lacking in virtual driving.
I know many friend IRL that are very smooth and quick driver around track ( experience with more than 400hp, rwd, awd and ff, driven in the wet and on street to slick tires ) but just average in driving games on pc and console. These kind of people would benefit most from this kind of cheating