Well, you need a decently high compression ratio to need to move away from pump gas. I assume by race valves you mean that they are larger than stock? If so, I would recommend a better cam, carb (or re-jet the original), and exhaust. That is the best way to pep up a 4 stroke. Those 3 things should be done together will improve flow drastically, but you will still be limited by the head itself until it is dealt with. The power gains might not be huge, but it will be noticeable. If you want to keep the compression reasonable, you could still go with a higher compression piston,, but it does not have to be extreme.
Currently : trx400fw, ct70, 250es,200x, xl80s, 350x, 350x, Trx250X,ALT185