I just switched to a K&N R-0990. Very slight performance upgrade, but it sounds nice, and looks great. You do need to play with the fuel air radio because it will be even more lean than stock. For me, I chose to shim the needles instead of going up on the main jet. If you slip an SAE #4 washer on the pins it should be perfect. That in itself is a performance upgrade, as the ninja 250 comes lean from the factory.