The simple but effective ("effective" as in "possibly minimal, but not counter-productive") bolt-ons: Intake tube (MIT or homemade PVC, etc.), long tube headers, free-flow muffler and a GOOD TUNE. Averaging, there's only about a 30 HP difference from a 5.3 and LQ4 (the "mild" 6.0).
It's all in what you want for your purposes. My gripe about the 4L60E is that there are only four gears and wide gaps between them. There are too many instances in my most common commutes where I'm going at just the right speed where it's too fast to downshift so it just stays in that next gear up and lugs the engine when trying to accelerate. At my current power levels, the 60-rating is sufficient so I can't bash it there. The 80 is stronger, but it also has different gearing that is more for low-end grunt than high- or top-speed driving. Still, it's only four speeds.
I would LOVE to convert mine to a 6L80E with all the factory stuff: Tapshift controls on the shift lever, appropriate gauge cluster, etc. It'd probably be easier to do a 6-speed manual conversion.