If I understand OPs question then he wants the mod that wll gain him the most HP, for me none of the above.
It would be headers, specifically long tube headers. Op take a look at this, dyno proven almost 23whp on a stock 370z.
http://http://www.ppeengineering.com...issan/370z.php I dont think the gen 3s, or any hfcs can give you that.
