So HotRod Magazine did a build up of what they thought was a junkyard 5.3L truck motor. The goal was to find the end limit of the LS motor. I'd say they did. The article lists all the details, and is definitely worth a read. I'll point out the highlights.
They took the motor, tore it down for inspection, and rebuilt it with pretty much stock internals. Hotter cam (but stock pushrods and rockers), opened up the heads with some porting, threw 75 lb injectors, a fast LSXR intake and a 102mm throttle body. Oh yeah, they also added two 76 mm CXRacing turbos.
Impressive build. The motor made over 60 dyno pulls (a good number over 1000 hp), and it sounds like they beat on it pretty good. It failed at 27 psi of boost, but the failure was related to the ignition, and when they tore the engine back down, found no issues.
My thoughts on the build though are mixed. They ran the whole thing on 118 octane (makes sense though for a drag engine). Also, the turbo lag must be incredible. That said, 1200+ hp out of a 4.8L LS engine is insanity. 4.09hp/ci. Shows how well engineered and robust the LS is. The smaller bore motors (4.8 and 5.3) handle boost especially well because the cylinder walls are thicker than their bigger bore counterparts.
My final thought is this. I've always thought the 5.3 (and potentially 4.8) is a better motor for an LS swap. You can pick up a 5.3 and a 4l60e from a truck from a junkyard for cheap (under a grand most of the time), and with some mods (cam, tuning, injectors, intake, etc, and not necessarily turbos), they return plenty of power for a street car, while still offering great driveability, decent mpg, and cheap maintenance. Considering junkyards LS2s and LS3s are a couple grand for the motor alone, I'd say it'd be significantly cheaper to build a 5.3 up.