Well, there's only a header on the H22 (as with all i4 motors).
It depends what you're building the motor for; supercharged or NA?
2.5" collector will net you more power if you're aiming for a built motor or supercharger, but if you're just looking at basic bolt-ons, stick with the stock collector size.
Btw DC sports w/ 2.5" collector, RMF, SMSP, Bisimoto, or Hytech (realistically, the Hytech replica) are all excellent NA headers.
Kamikaze w/ a 2.5" collector is little-known but probably one of - if not the - best H22 supercharged header.
