I really love our 2500HD. We tow a 7,000 lb. travel trailer or a 6,000 lb. utility trailer. There is no question that it can tow more. It is comfortable when pulling all day and is stable around big rigs and in reasonable wind when towing. Usually we just use tow/haul mode on the trans and keep the speed at 60. It could tow faster but this speed seems to be the sweet spot where it is most comfortable for me with our rig. We have pulled our TT over the Appalachians twice and the only driving adjustment has been to manually shift the gears and to duck in with the big rigs and keep the speed to 45 or so. We haven't pulled up the big mountains out west but would not hesitate to do so. I would also not hesitate to tow heavier as long as we could stay within the numbers. Our yellow payload sticker says we have 2,875 lbs of payload. With our family of 4 and the stuff we carry we probably wouldn't run out until the trailer is over 10,000 lbs.
Our configuration is a 6.0, 4.10 diff, 6 speed tranny, 4x4 with factory towing mirrors and a topper that sticks up a little above the cab. It isn't designed for gas mileage. Most of my driving is around town and overall I'm averaging about 11.5. Towing we get the same as others have noted, 7.5 to 8.5.