Next week my Hypertech Max Energy will find it's long, slow way to Alaska, and I'll let you know what it does. According to Hypertech's Dyno, the 6-speed gains +176 HP and +289 TQ from the '05-'07 Cummins with a 6-speed. A bit less with the automatic at +113 HP and +185 TQ. I've read that guys have put the level 3 (highest) program in and have achieved a 2-3 MPG increase. I'll have to see this for myself, though. I sold the idea to my wife on the grounds of MPG increase, as diesel is $3.39/gal up here in North Pole, Alaska.
Best price for it was $319 at 4wheelonline.com, saved about $60.
Give me a few tanks of coal and a few days of driving and I'll let you know.