I am considering buying a ZC Cylinder head that has been ported & polished, and has 3 angle valve grind on it. Do you think this would be a good investment and how much more hp's could I expect?

 

If the work was done by a respectable shop, then it's worth it. Your mods will benefit more by having the engine breathing better. But remember, the key word here is, was it done correctly and by whom? I'd say 5-10hp depending on your existing mods.