the dump valve is designed to work with a turbo, keeps it spinning by releasing pressure when you change gear - you can get some noise generators for non turbo models though
http://www.ebay.co.uk/itm/Saber-EDV-V3-Fake-Dump-Valve-Fit-Subaru-Impreza-MR2-/150657617376?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2313e4cde0
I would stick with just a cherry bomb / exhaust system and induction kit, that will sound great