Im not a expert by no means, but i have tryed to up the performance on my 07 XP and i have come to the conclusion that with my set-up the heavy cab, that as far as top end im getting all i will ever see. Ive got the clutch, the new ecu, the belt by dayco and from a dead stop to top end is great it will get there real fast, but top end is still 43 mph at the best and ive never hit the rev limiter on mind at top end, the only way i can hit rev limiter is in low gear.