save your money, the gains from a manifold are minimal without cams and a remap. by which time you are 3/4 of the way to a supercharger with another 80+ bhp on top of cams and manifold.
Flowed head, balancer delete, lightend flywheel and full exhaust with a sport cat would be my first steps into tuning a z22se.
i already have a exhaust system to fit, am investing in a sports cat in the next few weeks. Balance delete is also on the to do list in the new year. Was thinking manifold then cams when i have a bit more ££ next year and then remap it all at the end.