I have a chamber like that but have never run it. I have actually seen quite a few lately in the last year. Everyone I ever talked to said they were a top end pipe.
Coffman also had a pipe just like it with a resignator and you remove the water box. A friend had one on a 750 SX. It wasn't too awful loud.
There was also a FPP pipe for a 750 XiR ski. It was a center bleed that looked to the 750 SX mod pipe you guys are thinking of. Basically what a Blaster mod looks like. But it was shaped a little different. I want to get one to try out too.
Just to see which chamber is best. Really they all look the same. A stock piep is as good as a bean can. Any pipe is better than one of those! I can't believe Group K uses a stock pipe on the 850 big bore kit.
DBR