I stumbled upon this it is very nice read. I havent seen this posted before I maybe wrong.

http://www.scribd.com/doc/104227/How-Subarus-Factory-Boost-Control-System-Works-v11-01

It looks like the Cobb PDF from Cobb Forums.

It gives a good description of the physical mechanism for boost control. However, if you want to learn how to tune turbo dynamics, I think merchgod's sticky on enginuity was better.

It will be nice once enginuity comes back up.