Caffeinated...
Try setting your escudo downforce to minimum, spring settings at max, ride height at minimum. As for gearing set final drive at 2.000, set Auto setting at widest, then manually tweak each gear to maximum EXCEPT for 6th which should be set at 1.073.
Of course add the stage 4 turbo, and run it around the test course. Pick any race, and just run the qualifying multiple times, each lap checking your settings, until you hit the break in period (Rumored to be around 200 miles). This should peak your HP at 1881HP.
I've managed to hit 323.1mph on the max speed test, will all four tires on the ground. I've heard of other hitting 326+mph.
When you get bored, try out the other 180+ cars in the game.
AO