Contact these guys and ask them.....
http://www.blowerdriveservice.com/techcharts.php
This is a chart showing the boost you can run with a given c.r., but I don't know about propane. I think it is the equivalent of 104 octane fuel. If that's the case, you could probably run quite a lot of boost, but I would be concerned about the motor spitting the head gaskets out if you don't o-ring it.