Tuning is key. I've seen several people run 300 whp on a stock block, without sacrificing reliability. There have been a few that made 400 whp, but none of those engines lasted very long.

Using a thicker head gasket isn't the best way to lower compression, you might have a problem cracking the gasket under boost. With a good tuner, you can use an OEM headgasket and keep your CR at 10.6:1 without problems.