Considering that you use fair (comprable) cars to the "typical opponets"
Grand Valley 300: Yes, All 4 should be Lvl 25
Roadter 4h: At least 3 should be level 26
Laguna SCA 200: I used 3 driver, but only 2 were level 27.
Indy 500: at least 3 of the 4 drivers should be Lvl 28, but not below level 27. Strenght wear is important for this race and you can't post fast enough laps to win with a tired driver (trust me, I know)
Suzuka 1000: Nope. I did fine with 2 level 29 drivers and 2 level 28 drivers
Nurburing 4: Maybe, just do the A-Spec race first and win the Audi R8 LMS so it isn't a struggle (the R8 LMS, the Amuse S2000 GT1 Turbo, The RUF CTR12 YellowBird, and the AEM S200 are cars to watch out for as the race progress.
Tshbuka 9h: Have at least 2 level 33 drivers and a level 32. And use the Amuse S2000 GT1 Turbo untuned. You come out at least 2 laps ahead of the next Amuse S2000 GT1 Turbo (no, not the Amuse S2000 GT1 '04)
24 h LM: YES. HAVE ALL YOUR DRIVERS BE AT OR ABOVE LEVEL 35.
24 h Nur. Not sure, but I expect 2 lvl 40's and 2 lvl 39's to get the job done.