First off i'd try a fan on your current setup and see how it does.The 8xl would be fine on 5s,or you could use the 4s packs you have with a 7xl which would be cheaper than buying good 5s packs.Either setup should do well,the 4s obviously the lighter.they both put you in the 33-35000 rpm area.I'd try an 11/46 or 10/46 gearing and go from there.