Will a K&N filter make a difference on a stock
 

2002 CBR 600F4I with a m4 slip on? I mean anything noticable or should I not bother?

Save your money. Not worth it, IMHO.

Haha, I dunno. I installed mine right after a ride, then took the bike out again. I noticed the bike seemed to breathing a bit better. Dont buy it from K&N though, I got a brand new one in the box and wrapper for 20 bucks off ebay.

All Ive got is a slip on as well and I put a K and N in the air box. I didnt notice a difference, but its an upgrade either way you look at it. it might not be noticable, but k and n's last longer and work better for your engine.

reusability will save you money but for performance you'd need a power commander to exploit the greater air intake ability of that filter

I have a D&D slip on and a PCIII, so I get more out of my K&N than you would. That said, I noticed the bike seemed to breathe better and had a very slight increase in its "snap to" to when I rolled on.

Could all be in my head to, I never actually tested it, the results would be so small it probably wouldn't show.

Most places that have computer tunes pre-mapped will list them as "with or without aftermarket air filter". It dosen't make enough of a difference to tune for it so you won't feel anything either.

There's some historical data around here suggesting that the OEM filter works better performance wise. I'd ask for a dyno run to be sure.