A good friend with a 250f told me that either mxa or dirt bike did a shoot out of all pipes. I believe the Yosh had the best power but high $. The best value was the Dr. D, which my friend bought (loud!) and he's happy with it. Go check the library archives. It was just a couple of months ago.
Personally, have a Big Gun on my KLX and love it. Runs great, fit perfectly, and priced right. Know of others using their stuff and are very happy. Give Big Gun a call...they are very happy to talk bikes there.
I second the vote on the Biggun, I have one on my WR250F and I'm very happy with it. They have a nice look with a new end cap with a standard spark arrestor. There is way more bottom and mid and didn't take any top away that's for sure! I never tried mine with the stock header though. Dirt Bike however, did a pipe shootout and said the pipe felt better to the riders than it showed on the dyno (same as stock).;)
The Yoshi pipe is really good but expensive. I rode my then stock one and swapped with guys with Yoshi and Procircuit. I got a fantastic deal on a Procircuit T4 and I am very happy. Noise is up a fair bit though!