You will see a minor improvement with the exhaust (will sound a heap better though) & air intake done, but from experience the beast thing you can do is get a decent intake manifold. The standard HO one's are crap.

BTW, I wouldnt waste any money paying to get work done to the stock heads. They are crap to start with and can only have so much power extracted out of them no matter what you do. The most I would do to them is a home port job that costs you nothing but time.