I have a 2006 2500 HD GMC. I comes stock with 6 speed Allison. I haven't transfered my hitch to it yet, but putting it in tow mode and you can feel the tranny doing a lot of the work for you. Down hill or braking for the off ramp and you can feel the transmission downshifting to assist. The manual mode is something I look forward to. Put it in manual and you choose the highest gear that you want it to run in. What ever you choose is the highest gear it will go up to. That will be nice for long uphill grades.
I've heard nothing but great things about allisons and they do put them in A1 Abram tanks so they must know about heavy loads. The sales man said the Allison factory rep said the Park pin on the tranny is rated to hold back 17,000 lbs. Not that he recommended it, but he said you could park it on a hill and not set the brake with confidence.