My semi-informed opinion: (I went through a 5.3 to 6.0 swap last summer)

1) I would say somewhere on the order of 50 more hp...possibly 40 more ft-lb. Would depend on set-up...

2) My truck is a DD as well and I lost a good 1.5 mpg with the swap. Depends on driving habits though.

3) Stock tranny may survive all the towing but you would be better off going with some kind of shift kit just to be sure.

I can tell a definite "seat of the pants" difference between the two, although I don't do any towing so your experience may be different.