My advice for La Sarthe I: Use your R8. First things first: DON"T BUY ANY HORSEPOWER UPGRADES: Your R8 is good to go from the start. Just change the oil to pump out more horsepower if you haven't already and get R1 and R2 tires. Start the race on R1's all around, then on your first pit stop, switch to R2's.
If you plan on B-Specing part of the race, keep him on setting 4. With R2's all around and setting 4, your B-Spec driver will leave everyone in the dust. Just make sure you have high B-Spec points.