don't look at full-Mx bikes as a comparison. just as you wouldn't look at full-GP-2-strokes as a comparison.
nobody uses corrected compresiion ratio. jap's normaly give that number, not the geometric. but any tuner will tell you it's useless, they'll never taalk about corrected compression.
only looking at the compression ratio is pretty meaningless. an expanson-pipe is a presure-device : it works like a turbo : it pushes more mixture into the cylinder than its geometric volume so it raises the actual compression.
good squish is very important running high compression's. you need a tight squish-height : where there is no space, there can not be mixture that can detonate.
high compression's are not all good for power. High compression-engines uses more of the energy from the burning to push the piston down, meaning that there s less energy in the exhaust gases, and that means that the pipe will work not so good. It's even possible that your pipe will be to big : by the time the gases get to the belly almost all energy is just to suck on the cylinder, and no (or not enough) energy is left to push fresh mixture back.
dropping the compression will uses less energy to push the piston down, the pipe will work better, suck harder and push more fresh charge back into the cylinder resulting in a bigger bang the next time, and that biger bang ...
but it all hangs together. changing one thing that normaly is supposed to be good for power can result in a drop of power, as the rest of the engine ain't working well together with the change.
it's not easy. I've just tested 6 heads (up to 15.7:1) on my MX-moped, with 2 cylinders and 3 different pipes to get plenty of data. made 2 straight test-pipes, each one more power than what I had. made a curved copy of the best pipe and lost 15% of power ...