You use 3rd only at turn 1 and 2? I know I'm doing something wrong in T1, so everything is worth a shot.
I was using 2nd gear there.
Where you shift depends entirely on the power curve, when it said peak power at 5,700rpm I had to go into an arcade race and check the power, and it pretty much holds that peak power until the 6,600rpm fuel cut-out. If you don't know what the power curve looks like it's very hard to tell when to shift. I remember in GT4, the Hemi Cuda I think had peak power some 2,000rpm or something before red-line, and VERY sharp power drop-off after that peak power, so if you couldn't drive manual it meant you were losing seconds per lap.