I've got a 2004 400EX and I'm not sure which slip-on to get and I have a K&N still in box here are my guidelines.

1.Has to be pretty quiet (stuck-up neighbors)
2.Great Low to midrange power I don't care too much if I lose a little Top-end.
3.Slip-on only I don't want a full system.
4.Not over $250

 

From my experience,

I recommend switching to a 14 tooth front sprocket especially if you aren't worried about top-end. Forget about a slip-on if your are not changing the headpipe. You aren't going to gain hardly anything unless you just can't stand the looks of the stock slip-on.

Open the airbox lid and get a Dynojet kit. You will gain a couple of HP. It is a noticeable power increase and will only cost you about $50.

Save your money for:

1) new front shocks

2) new tires

3) stainless swingarm skidplate, or

4) new handlebars

/NotuRaptor