Compression

You have to think about it the other way around, compression selection is an output based on the other choices you have made in the build, as @AJ/FormS was eluding to. You need to make sure you have a cam with enough duration (which is actually where your RPM HorsePower increase comes from) to keep the cylinder PSI to a reasonable level. What the compression really does (as @Bewy said above) going between 9 and 11 with a long duration cam is keep the cylinder pressures up near optimal so you don't loose low RPM torque. If you put a long duration cam in without increasing compression the cylinder pressure drops off and you may gain if you are running it at high RPM all the time (like at the track) but once you get back on the street you have lost a lot of low end torque because the cylinder pressures are so low. It is a delicate balance that you have to design depending on what you are after.