Get an HMF, it is by far the best buy for an aftermarket exhaust using the stock header which will retain the bottom end.
Lot's of great info here for all exhausts that fit the KLX.

 

I ordered Q4 i hope i will like it, i stil havent did any carb work.. not sure if i will, i just cant see me... busting out the ol' carb and rejetting it.. lol , regardless the bike should improve, so i'm really excited, also have my first enduro next sunday , so sporting my new Q4 will be awesome!!!! Thanks again for the help n stuff

 

I have a FMF Q4 exhaust and power bomb header on the way. When I looked into it before only the pipe was available now that the header is available I ordered the whole system direct from FMF. I've got a 330 kit on the way as well and hopefully a new carb in a few days. I will let you know how everything fits. I didn't want my bike to be too loud and the FMF claims a little more hp with the header so what the heck, its only money.

 

If you don't rejet, don't enlarge the airbox opening or you can damage your engine.

To get anywhere near the full potential from that pipe, more air intake and proper jetting will have to happen.

If you choose not to, most of what you will gainwill be in the form ofbetter soundand looks. Try tofind someone who's doneit before and have them do it or show you how.There's also lots of info right here in this forum.I hate get on my soap box, but that's a good chunk of change to drop on a slip-on and not getthe best it has to offer

 

In order to reap the full benefits, IMO you should either get the new FMF header or FSW header to go with the exhaust. If you ever decide on a FSW header make sure you tell them you have an FMF exhaust because he makes them a little different for them. You also will need to open up the air box and re jet. I'm not sure of all the smog stuff on the 250S as I have a 300 but you should also eliminatethat as well. I would change everything out at onceas better exhaust flow will also require more intake and jetting changes to make it run correct. The jetting on the CVK is actually pretty easy to do. The hardest part will be removing and installing the carb.