Most of the aftermarket head gaskets are a tad thicker than stock to make up for taking a few thousandths off the head. If it is really bad, Felpro also makes a spacer, tho too much warp will cause the cam to bind even if it is shaved flat on the bottom. Some shops these days can heat and straighten aluminum heads nowdays though, so that may be an option if it is very bad. Here is a snip from the factory manual:
Quote:
With a straight edge and a feeler guage, check the lower face of the cylinder head for distortion at six points as shown in figure 6-50. (Shows straight edge bar on the head with a feeler guage slid under it) If the amount of distortion is more that 0.2mm (0.0078") but less than 0.4mm (0.0157) correct with a surface grinder. The cylinder head should be replaced if distortion is greater than 0.4mm (0.0157")
As for how much can you shave to add performance? Its not too much, but there is a little space in there. The valve to piston clearance goes down though, so floating the valves starts to become more of a problem. If a valve is still open at all when that piston comes up, it will hit it. You might look at
http://www.hotgemini.com if you are really serious about squeeking every last bit of power out of that motor. They do some crazy stuff with Isuzu G180 motors in Holden Geminis down there.