I run K&N's with Extreme tops. As for carb sizing there is a simple formula for naturally aspirated motors;

Required CFM = CID X RPM / 3456 X VE (volumetric efficiency)

If you don't know VE then assume 100% max unless you have an exotic setup where you can see 110% (high rpm's, long duration cams, higher compresssion, geat heads,etc) . Most engines are only about 85% efficient though.

I've actually seen test results (Not from K&N) showing MORE Air flow through an air filter set up over nothing.