valves don't increase combustion. Pistons, rods, crankshafts, cylinder heads increase combustion. If you are going to go with a thicker head gasket, please get a longer timing belt. I'm sure you can get the belt to squeeze onto the cam gear, but the belt will have a lot more strain on it and could possible skip a tooth or even worse, break.(not cool) If you are talking about cams, those will give you more horsepower, but not compression. Even if you do get cams, there is still a lot you need to do to get the most out of them. Cams change the time your valves are open and the time they close. I'd defenitely do some research on timing and proper valves and plugs because if you do the setup wrong, you are just going to burn the craep out of your valves and eventually destroy your engine. good luck.

If you are going to go turbo, i'd get your head port and polished. That will give you a lot more power and reliability then just valves. Valves are usually changed to help with reliability and strength, not horsepower. Just changing valves without port and polishing isn't going to do much.