I sold my 11 5.3 Z71 a fews months back. But recently ran across a rare 09 LTZ 4x4 with the 6.2L and only 11k miles on it, its one owner good clean history report. Bought it last week sight unseen off the net, its over 1000 mile away. Should have it in a week or so.
I have never been all that impressed with 5.3L and have compared the 6.2 to the 5.3 back to back in the pickups, its night and day different for power and performance . However fuel mileage is not all that different maybe 1mpg in the pickups. One of my coworkers has a Z71 ext cab Sierra with the 6.2 avg mpg has been mid 14s, my Z71 ext cab Sierra with nearly identical options but with the 5.3L also has had an avg in mid 14s. My sierra window sticker said 21mpg his said 18mpg. Mine never saw more then 16-17 at its best same with my 5.3L Tahoe, pretty much both were always in that 14-15 range all the time mixed driving 10-11 if I was towing anything.
When I bought that 11 Tahoe I had looked nation wide for a low mileage 6.2 with no luck, finally got one. Cant wait till it gets here.
Good luck on your search, they are very rare and very hard to find with low mileage. They were extremely hard to find when they were actually still building them back in 09'.
I am now however a 100% believer in that the 6.2 is worth it after driving and comparing the pickups back to back and also having 2 friends now the 6.2L sierra pickups to compare real world mileage notes with.