I'd take the car back to Mastro w/ the boost gauge attached and show them it isn't right. I would attach gauge to the line running off the BOV as that's where Subaru recommends tapping boost readings for the OEM gauge. I would also consider taking the CAI and BOV off just to avoid making an asshat out of yourself if that ends up being the problem. Who installed the aftermarket parts?

I'd also kindly ask for my $40.00 back. (assuming it's really off and you didn't "hose" it up.)