I found this web sight on air filter's. I know most in this are automotive filters but it mentions the UNI and K&N. Might give you and idea how well they work. Just made me wunder more.
<a href="http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm" target="_blank">http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm</a>
<a href="http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm" target="_blank">http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm</a>