Well, last time I tried, I got 6 out of 8 done. It just takes practice, practice, practice.
Skip S2 (GT40 at Seattle) until you've done everything else except S8. All S2 does is frustrate you and make it impossible to beat the others because you're so wound up.
I've heard the way to beat S8 is to keep the GT1 in higher gears to limit wheel spin.
Good luck! I still have those two to beat, myself.