I myself had a gmc single cab long box with a 8.1. You are right they are shitty on gas, but they are not any worse than the dodge ram 1500 i had except the 8.1 beats the shit outa the 5.7 in the dodge. as far as horsepower numbers go the 8.1 is right there with the 6.6. but the 6.6 has it on the torque side. What I loved about the 8.1 was the instant torque. No turbo lag. I loved pulling trailers with the 8.1 as well, it pulled just as good as my dmax does. and it sounded awesome in t/h mode when downshifting. the downside is the 8.1 does not have that awesome whistle when cruising through the parking lots or down the city street. There are ups and downs to both though. I would still buy an 8.1 for a toy but i would never get read of my dmax. and with fuel prices and gas prices right now it is probably pretty close to the same to run either one. I only average 12 to 14 in the winter with my dmax. 15 to 16 in summer. that is all city of course. the dmax is also easier and cheaper to upgrade to make more power. and a bigger aftermarket behind it. sorry for the rambling.
2007 GMC Sierra 3500HD 4x4 CCLB DRW LMM
DRW Club Member 193 Mods: N-Fab wheel to wheel nerf bars, resonator delete, pcv reroute, Egr block plate, 3" MBRP downpipe, 4" MBRP straight pipe exhaust, Edge Insight CTS, EFI Live DSP5, Air Dog II 165, Stock fuel filter delete
2013 Victory Cross Country Mods: power commander, victory performance exhaust
2012 Chevy Cruze 6spd manual
2015 Toyota Sienna (girlfriends)