1. The stock post 1980 air box is better for power than the old round one.
2. The K&N filter for said post 1980 airbox does not fit well at all. They fit so poorly they allow unfiltered are past it.
3. If your building a cafe racer or fun weekend hot rod have fun with it. You will feel no power loss or gain running individual filters. You will have more intake noise which I personally like.