Hey buddy the Duramax willl walk the Dog on the 8.1L. My girlfriends father had a 05 2500 C-Cab 4x4 with the 8.1L 496ci. with the Allison, with 4:10's. He added a Superchip, Flowmaster, and K&N. And it was still slow as pond water, but sounded good. Plus he was getting 8-10 MPG on a good day. He got rid of it and got a DURAMAX, put Chip on it and it will SMOKEEEEE his 38's Mickey Thompsons. Duramax is twice the motor the 8.1L ever thought about being.
My fathers C-cab dually with just chip and 90gal. fuel tank full in back. Ran a 9:40 in the 1/8, 14:8 in 1/4.