Few comments:
1. The Laguna Seca that in GT4 is pre-MotoGP. Since beginning of 2005, they have changed the curbing to from tiger teeth to flat to satisfy MotoGP. In result, it is at least 1 sec faster around the track.
2. The difference in GT4 and real life like mentioned by Clarkson is braking like in T2, T5, T6 and Corckscrew. In real life you would have to brake much ealrier.
3. Also, it is possible to take T6 almost flat out in GT, where it is impossible in real life.
4. I think 1:41 in a stock NSX is a tall order. Maybe if Ayrton Senna drove the car......