You can do type r japanese cams, ctr or itr non us spec for good gains, any bigger you will need valve springs and retainers also. An engine management system would be nice as well, but just some itr cams alone will be more like what your sounding like you would want. i say with tuneing (with a vafc or like a hondata also) 10 whp or so. possably more. (like 5 at the most.)