AndrewF, check out tstproducts.com. Their Powermax box includes an in cab controller that lets you choose from stock up to level 9 power levels at any speed along with showing you exhaust temps and turbo boost so you don't have to go out and buy seperate gauges. Level 9 added 140HP and 360LBFT at the rear tires on the bone stock 6 speed truck TST did their testing on. I bought one of the originals back in Feb. for my 6 speed truck and have been very happy. Within the past couple of weeks they've come out with an improved version that has more and harder hitting power at lower rpms than the original version. New boxes are shipped with the new program and original customers can have their boxes reflashed to the latest version at no charge. Greg at TST will answer any of your questions.
2003 GMC 2500HD 4x4 6.6 Diesel <br />443+HP/944LB-FT unstacked #2 only