the diesel will outpull the 6.0 all day but the 6.0 wont be bad either. what gearing does the suburban have in it. 4.10's would probably be a good upgrade if it has the 3.73's. unfortunately I havent done a lot of heavy hauling in my 6.0 but it has done well when I have towed with it. after I lifted and put 35's on it struggles a bit on hills since I didnt jump to 4.56 gears but as you stated before just need some more patience on the hills.