OK, I think I've figured out how to win reliably with the FGT.
1) Setup is important. Do/set the following.
a) Rebuild engine (500Kcr). Chassis rebuild is completely unnecessary.
b) Auto transmission
c) Racing Hard tires (looking for hardest possible -- I tried the rain tires too but Racing Hard is best)
d) Downforce minimum front and rear
e) Ride height MAXIMUM front and rear (+10mm)
f) Transmission set to 242mph max
Optional:
g) Springs/swaybars full stiff
h) Whatever driving aids you want (I leave them all off)
2) Get the right field of competitors. As mentioned, the Jag XJR-9 is unbeatable; even the Ford GT LM Test Car and the Zonda can be pretty dicey (esp. when they both appear together -- they'll draft each other). Exit and re-enter till you get a field you can live with.
3) Use the minimum amount of steering to keep the car in the wall. I've gotten it so that it actually comes slightly off the wall in the "short chutes" btw T1-T2 and T3-T4.
This procedure gets me consistent laps in the low 49s without any drafting (even saw a 48.9 once). After the engine's worn out it goes up to mid/high 49s but depending on the competition it's still enough to win or at least podium.
Hope this helps! For those about to grind...we salute you!