You can easily get over 500 flywheel HP with a head/cam LS1. See this link, for instance:
http://www.airflowresearch.com/ls1_dyno.php
That's still a pretty mild head/cam combo, idles like a stocker. I ran that exact combo in a '71 Camaro for several years - it embarrassed its share of late model sports cars on the road course before it blew up (oiling problem).