The stock intake manifold on EFI Zs isn't as restrictive as you think. First get rid of the restriction in the head, then it might be worth it to look at the intake. Simply trying to cheat the system by upping pressure or changing fuel injectors when putting in a bigger cam won't work, at least not well. The fueling curve will be much different. Headers won't give you much of anything on a stock engine, performance increase from compression without changing anything else are also minimal as well as limited, port and polish won't do much either without supporting modifications. A lightweight flywheel, on the other hand, is a great upgrade! It's not as easy as it sounds. Basically, if you want to keep stock EFI, it's not worth it to heavily modify the engine. If you plan on serious modifications (e.g. headwork, cam, etc.) then either triples or standalone EFI give you performance and ease of tuneability. There are other ways of improving performance without doing engine work, besides giving it a good tune-up and making it run as good as it can, including closer-ratio trans, lower geared diff, lightweight flywheel, big exhaust (keep stock manifold).