Take a read through this article: www.mrcontrols.com/primers/power.htm
It will give you a lot of good insight in to gaining big HP from the 3sgte. I've seen dyno charts of 700hp! but on the flip side friends of mine have also suffered a cracked block at 350hp so sometimes even though you do things right you can just be unlucky.
Some guys are in to the Charge cooler setup, they have a lot of positives but also note that the install and complexity is a bit more difficult than just using a good air 2 air intercooler. Take a look at ATS Racing and Berk technology stuff, it seems to work pretty good for the 3sgte.