I think you're correct regarding the V4/V8 drone/buffeting, but I believe there are two different issues. We have the V4 lugging/drone too, but our yukon also has the horrible buffeting sound/feeling at speeds of 40+. I can put truck in "N" and coast and still experience the buffeting which takes the V4 out of the equation. I believe we are talking about two different fixes. I plan on having the updated software done for the V4 and also having the roof issue resolved with the fix mentioned below.
The buffeting issue has been fixed by a dealer. When I say buffeting, I am referring to the sound / pressure feeling like when you have one window down. Supporting all roof bows to sheet metal and then lining the sheet metal with a sound proofing material has cured one case 100%. suburban was unbearable and caused head pressure feeling and owner claims it is completely gone now. If anyone has the roof box TSB done, make sure they attach all of the roof bows and not just the first one. I guess this is why the orig tsb is not fully taking care of the issue. The remaining roof bows near the back also need to be further supported.
I am guessing many do not have the issue where they are all detached and some do. Some may have only one detached while others have all bows detached which tells us why some notice the buffeting more than others.
I even experimented with our XL and loaded the roof with medium weight (multiple pieces of luggage strapped to directly to roof) and the buffeting feeling was much less and the overall sound inside the cabin was different telling me that the roof sheet metal has a lot to do with it. However, there could be other items ,driveline, mounts, ect that play into but I believe the sheet metal is what is making all other issues even worse.