It depends on whether you use your new power for good or evil. Those that use the propane sparingly report mpg increases of 1-2mpg, primarily on the freeway. Those of us that turn up the flow don't see a real change in the mpg, but have a lot more fun than before.
I've got the Powershot2000 system and that is available through a number of dealers. Rockin-S-Performance is one of them. TYMAR is another.