I have a BlackBear tuned 6.0 in my '00 Yukon XL 2500 and it made a big difference towing and unloaded. Unloaded it has plenty of power and averages 14 mpg city/hwy combined. Towing my ~8000lb TT is a much better experience as well. Prior to the tune I pulled several hills in 2nd with a lot of throttle. With the tune I can keep it in 3rd longer and if I have to pull it down to 2nd less throttle is required.
I do have a ebay 'cold air' intake and the exhaust is slightly more open, but the tune is by far the best 'bang for the buck' towing mod I have done to the Yukon. In order to maximize the benefit I have to run 93 octane fuel, but the benefit outweighs the $0.30/gal price difference. Last camping trip I averaged 8 mpg, and while not stellar, that 8 mpg was towing 8k @ 65-70 MPH loaded with 2 adults, 3 kids and gear for all 5 while blasting the A/C. Not bad IMO.
Also, the 'cold air' intake does lower intake air temps on the highway to 2-3* above ambient temp as verified by my ScanGaugeII. I can't speak to that adding any more power, but it does do what it's supposed to.
One more note - if you decide to get the BB tune, make sure your ignition parts and sensors are good to go. The tune will make any issues you have more prominent; I had to replace my knock sensors after the tune because the new PCM was asking more of the sensors than the stock tune.
'00 GMC Yukon XL K2500 - 6.0L, 4L80e, 3.73s, P3, AirLift, BlackBear tuned.
'14 Aerolite 282DBHS - Andersen No-Sway