I'm gonna replace the air filter and wondered what peoples experiences were!! ;)
I wouldn't think you would notice any significant difference between a paper filter and a K&N. Worth a try though; ECU will automatically adjust for the improved flow. The stock airbox has a restriction in the plastic pipe to the charger (something like a 40mm hole in a 60mm pipe). It makes a big difference when this is removed, particularly if you run a smaller pulley. When I put on a 69mm pulley with stock air box the peak boost was almost the same as with the original 75mm pulley. Cone filter (e.g. K&N 57i) makes a noticeable difference but that's mainly because you lose the air box restriction.
if its my old car - i supplied a stock and a de-restricted air box when i sold it!
but i like stock paper filters unless very highly modified!