IMO the 5.3 does just fine. Would a 6.2 be better? Well hell yes. But is it worth the extra $10,000+- to you? That is the question (and yes, I know you get a lot more upgrades with the extra cost, but that is not my point here. The point is, that is the up-charge if you want the motor). I do not want to argue about the fact that the 6.2 is a much more powerful motor and better suited for a rig of this size, because it truly is (not really an opinion, it's pretty much a fact). We have owned a Denali before, so I do have experience with both. The 6.2 effortlessly pushes the boat around in comparison to the 5.3. But, that does not make the 5.3 a worthless turd that has no belonging in a rig of this size.
My family and I just got back from a few thousand mile trip across the country, consisting of mainly 75-85 MPH highway speeds. The Yukon was packed to the gills with a week's full of gear for the four of us. Many mountainous areas were passed, with long uphill stretches that seamed to go on for miles or more. Never once did I feel like the 5.3 was struggling. Did it downshift once in a while on real steep climbs? Yep. But nothing that was an annoyance or out of the ordinary. Most of the time it just chugged along in overdrive with no issues at all. It averaged right around 21MPG for our entire trip, with the higher speeds killing it quite a bit. At one point the average was over 23MPG, and the highest "last 50 miles" reading was over 25MPG (easy, flat, non-stop 45-60MPH section of road lasting for an hour or so).
If the 6.2 (motor only) was an upgrade option for the regular Yukon's it would be a no brainer. But, as in our case, adding another $8-10 grand over and above a fully loaded SLT to get the motor simply was not worth it to us.
Anyways, just wanted to share with you my opinion and recent experience with our 5.3. I think it is a wonderful motor with plenty of power for the size of the vehicle. We have absolutely no regrets not getting the Denali.