I've had mine (2004 Silverado 2500HD D/A) for 2 years and love it. I get 18-19 on its own, and 13 or so towing my 25' Safari. Except when towing against the wind in Kansas. Then I could watch the fuel gauge drop.
As for hp and torque - it is the 6.6 liter, don't know which engine, however. Like Over59, the danged thing will idle itself uphill. Plenty of torque for me.