If you get serious about making some power with your 5.3, there are some very good heads available from TFS that were developed just for that engine. They are the GenX 205 and offer 58cc combustion chambers, so you have a compression increase instead of a compression drop, like all the factory heads deliver. Compression is a very important component when it comes to performance. The flow improvement is impressive, with the intake @ 284 cfm @ .600" lift and the exhaust @ 228. Compare that to the stock head @ 231 and 193 at the same valve lift.
These heads are designed to work with the small bore of the 5.3 and that also is critical. That's another reason that the other factory heads performed poorly. The valve shrouding from the small bores impacts the flow of the larger factory heads. If all the factory heads were tested on a 4" or larger bore, the outcome would have been different.