As stock, the FZS runs on the rich side. So you can stick a standard K&N filter (staright swap from OEM) in without dyno time, and it will improve performance a bit - but most of the advantage is the lack of maintenance needed.
Pod filters on the other hand are a different kettle of fish. So far, I have only heard of one firm (in London) that has put the K&Ns on a Fazer and made it perform well. For some reason, those Mikunis are not keen on them, and it won't run too well if you just bosh them on.