Other than elevated silicone reading in my UOA and a slightly nicer low-end (low rpm) grunt (sound wise), I trashed my K&N after a year of usage in my Mazda 323 back then.
I wouldn't bother with air intake or filter mod anymore, citing (a) almost no gain in performance (OBD-II ECU are very controlling, and can compensate for your K&N replacement), and (b) the ever-concerning elevated silicone readings in UOA.
Q.