I have a Yukon Denali, no the XL, with the 6.2L and Max Towing package. I tow a 25 ft. trailer, about 6700 lbs loaded. That's about the limit for my vehicle. With the longer wheelbase your XL could probable handle a longer trailer (assuming WDH and antisway) and a little more weight. However, as mentioned, most SUVs and pickups reach their payload/cargo capacity limits before they reach their towing limits. And as I recall, the XL usually has a slightly lower towing capacity and payload capacity than the regular Denali.
Oh, and if you have P rated tires and are going to do much towing I would consider a higher rated tire. Not as nice of a ride in town but more stable on the highway. I changed to Continental Terrain Contact H/T that is close to an LT and can handle inflating it more.