he will lose power with ITR header (USDM/JDM) since the GSR doesn't make the compression an ITR does and lacks the airflow needed for that header. HondaTuning did a dyno test with headers and the ITR headers both showed power loss through entire powerband even with a tune. So unless you up the compression on the GSR the header won't do anything good.

Don't expect huge gains without internals. Once you get the bolt-ons you want look into buying a VAFC and getting a dyno tune. I had a GSR with spoon intake, RSR header, buddy club exhaust, invidia cat, spoon throttle body, rebuilt tranny, and stock internals only made 165whp with light tune.