500HP is very easy to achieve. A set of ported heads, Valve springs and a cam is pretty much all you need.
A few other things to add to the list to improve reliablility:
Good Quality Rod Bolts, these are a bit of a weak link in engines turning really big revs. The Rods are very strong and there are plenty of blown and turbo combinations making 600HP on stock rods.
Ported LS6
oil pump. Early motors ie 97-00 had some
oil pump failures so it's a good idea to replace it with a later pump. Porting it improves flow and they usually have the pressure relief shimmed to up the pressure a bit.
I've got a new cam, dual springs, titanium retainers, molly push rods, LS6 Pump coming from the US. Guys in the US using this cam regularly get more than 400 RWHP and with ported heads are getting up around 440RWHP. That equates to well over 500HP at the crank.
Cheers