Subarus normally have recirculation valves so some of the air gets pushed back into the intake, the maf will have metered for this, when you fit a vta dump valve obviously the air escapes causing the car to thing it's still getting the correct mix but it isn't. It's a bit hit and miss people have reported problems and gone back to oem type while others run fine?
I'd change the gasket and have a look at all your vac lines from the turbo to boost soliniod, there should be a brass pill inside one of the lines (not sure of location) on newage cars and may be worth cleaning out the boost soliniod too with some brake cleaner.