Plenum for the Diana Skyhawk/ Artemis P15 (.22cal)?

If .22 standard port should be big enough (It should be around max 80% of Bore/ Pellet Size, if larger the pellet can get caught or damaged in port when loading), hammer spring can be changed, barrel length, plenum volume, reg setting, valve stem diameter, valve poppet shape all can contribute to power increases...There are some good threads by mackeral5 over on GTA on mods to P15...