My 07 5.3 has 135K on it now and has been running like a top. It does have a lifter tap on a cold start if it sits for more than a couple days but it gets quiet fast and has done it since I bought it with 40K. It also gets 15/40 Rotella just like almost everything else I own.
I refuse to put 30w water in that engine. Some of the GEN IV's do have oil consumption problems that stem from the AFM system but that's typically an easy fix.
As far as replacing your 5.3L with a 6L, it's an easy swap as long as you get the same generation engine, it's a direct 1:1 swap but you will need retuning. It's a little more wiring but you can put an older (99+) engine in your 07 truck. It's been done. As well as the other way around. I'd be concerned with the 4L70E taking a dump before that engine does though. I have a backup sitting wrapped up on a crate for mine for whenever it decides to go south. With those things, it's when, not if.