I would say the 3rd row is pretty much not usable for adults unless you have the second row captains seats. Then they can put their feet between the 2nd row. I don't have an XL so I can't comment on it's space but I think the floor is higher (under your feet) in the regular Tahoe or Yukon because the tunnel for the rear axle raises the floor (making it so you can't put your feet under the 2nd row seats). So I think it may be better in an XL because the axle should be farther back and the floor should be lower under the 3rd row peoples feet.
When I was looking at an XL vs. a regular Tahoe they told me the seating space was the same and all the extra room was storage behind the 3rd row. Not sure if that's accurate or what you're looking for but hopefully it helps.
Hopefully that all makes sense.