Anyone with the stock box and AFE PRO 5R Filter?
 

Would like to know how it compares to the K&N one

It's probably much better since it is a dry filter (no oil on the MAF). But as far as filtration who knows since it has likely never been tested interdependently.

aFe 30-10115 OE High Performance Replacement Air Filter Mazda RX-8 All 2004-2005

nope 5R is actually oiled, Dry S isn't

Just install a good OEM style WIX filter, there is no reason to run anything different.