I got the K&N FIPK on my 06 for over a year now and I got no problem.

 

I also put the FIPK in my '07......it sounds niiiiiiiiiiiice

 

I don't think there is much you can do to improve stock performance on the S2000, short of FI installation. I've used the K&N drop-in air filter for about 20K without any problems. My UOA's show universal average silicon ppm levels, so that would indicate to me that the K&N does not increase dirt ingestion in any significant way.
You do have to be concerned about over oiling the media after you clean the filter. It will, and has damaged O2 sensors if you do. It did this on another one of my vehicles. Now I just buy a new one when the old one gets clogged (20-30K miles).
Make sure you examine the filter occassionally (underneath and behind) for dirt.