So you have it set up for the wheelie trick, I assume? I didn't read the thread, but I saw "1151" listed as the top speed.
I ran the GT Allstars Pro in a 787B with Turbo 1 fitted, for a total of 946 hp. I beat the R390 by 1':13.367" with a top speed of only 263 mph - no wheelie trick.
My guess is your hitting the rev limiter? Are you accelerating hard up to a certain speed, and then bouncing down, up, down, up? If so, your gearing is too short. Here's what I do:
Drop suspension to lowest ride height
Set both Toe settings to zero - don't worry about camber
Set all downforce settings to minimum
Qualify - it's the only way to check you gearing. Leave your Autoset wherever it is, and move your Final Drive setting a bit to the left (taller gearing). Run a practice lap and see where the car tops out. At the end of each straight, in clear air (no drafting) you want to be about 750 rpm below the rev limiter in top gear when the car stops accelerating. If you are hitting the rev limiter, move the Final Drive a little to the left and try again. If you're well below the limiter, move the FD to the right (shorter) a little.
The point is to adjust the gearing so you're going as fast as possible without hitting the rev limiter. In Free Run, you can make the gearing a little shorter since you're never drafting. But in a race, you need to allow a little extra rev in case you're drafting, which can allow the car to go faster than it can in clear air.
