The secret is the fact that the Forza version of the track is so pathetic (a slightly narrower version of the PGR2 track) that there are many places once yo uget the rythme right, you can take flat out... like the whole back section up to the carousel.
I managed to do 3 back to back sub 7 minute laps in the final race to win the Porsche 962.
That was with the F333SP on stock setup using the controller.
You just have to realise the tarck and physics engine of the game allow for really unrealistic speeds on certain parts of the track.
You have to just get into the "Forza Zone"
the FM1 fanboys over at ForzaIs TheGreatestSinceSlicedBread.Com will have you believe the game is a simulator.. but its just really a slightly slimmed down NFS - fun but you must not approach the Nurburgring as if you would in a real car on a real track.
Just trust the grip is insane and the speds in some secitons are unbelievable. In that aspect the Forza Nurgurgring dissapoinrted me - compaired to say GT4's version.