I have a 2001 Freightliner 18wheeler with a Cummins N-14 in it. Before I took delivery from the dealer I had him take it to Cummins to turn the power up to an even 500hp. All they did is put a computer on it and push a few buttons. It had been reduced to 435 for the same reason you state. It was a fleet vehicle. Try taking it to Cummins first and turning it up with the computer. Most of the time these vehicles are offered to the original customers in a variety of hps and usually they choose a lower setting. Chances are you can increase hp, road speed, and engine rpm just with the comp. If you can't then it probably means it's been done already. Only after trying this should you try the aftermarket stuff. Sometimes you'd be surprised at the results. I was.