Run it in until you hit 1635 BHP, then set the front wing to 50 and the rear to 200. Don't ask why it works, just try it. On the subject of suspension... I can't help at all for high speed, sorry. I prefer to use the X1 as intended (going round corners extremely fast).
I was only able to reach 330 with mine, do as mistersafeway suggested then set the top speed to where you want it and tune the final gear ratio until you get the desired speed. that should help
put final gear at 4.650
put top speed at 311
bring the final as down as you want it
1st gear all the way to the left
2nd gear play with it
3rd play with it
4th play wih it
5th play with it
6th as far as you want it
