Ultimate super trucker rig?

The 'old' Cummins signature (non-EGR one) was a 600 hp version on the N14 (a heck of a motor, IMHO). The 'new' Cummins 600 hp version is a hotted up ISX (EGR). You only get that HP at higher RPMs and higher gears (cummins has some software to limit torque/hp in lower gears to prevent driveline damage), so really, it wouldn't have any adverse effect on the truck, except that it would have to have a big enough hood (and grille) for the radiator.
Odds are you'd get better fuel mileage, providing you drive it right, and keep your foot out of it (except when you should hammer on it).