I have no data on power losses, if any, from using some kind of "DB Killer".. MikeW has posted fantastic thread(s) full of information on noise levels/slip-on choices.. Search on him and his threads and be enlightened..

 

I prefer the 300 muffler over the PC4(mine was from a KLX300). The PC4 was super loud. It gave me a headache on a 40 minute ride. The bike did sound awesome. And power gains were noticeable, but are mostly, if not all there with the 300.
When I finally get the 351 kit, I'm going to get(if it's still available) a Four Stroke Works header for my 300 muffler. It is the width, or almost the width of the pipe in the 300 muffler, which neither the 250 or 300 header is.
You can remove the plug from the 250 muffler and make it a 300 muffler. Then order a muffler cap on ebay, which comes with a screen, and drill/tap some holes to mount it. Instant 300 muffler, spark arrestor intact(even w/o screen) = National Forest legal.
The 300 muffler/header set-up is getting harder to find though.

 

I agree that the 300 muffler/header setup is getting harder to find on ebay. I also can't find the OEM buttplugs/endcaps/baffle for the 300 muffler on ebay anymore. I ordered an aftermarket one called something like "CNC 3D Spark Arrestor cap".

I did just successfully install the OEM KLX300 muffler on an even cleaner (almost pristine) 250R Header pipe. Haven't got to ride yet due to the weather, but can't wait since previous owner of this bike definitely jetted it for aftermarket/uncorked muffler.

So that's a tip for those looking to go this route - remember that the 250R/300R exhaust parts are interchangeable. If you can't find a 300 header, maybe you can find a cheap 250R header like I did.

 

That's the cap I was talking about.
The spark arrestor is in the pipe, I'm pretty sure. The screen is dressing...I could be wrong about that though.
Good call on the 250R header.