blow off valves release pressure off the intake when the throttle is closed. it releases the air into the engine compartment, and really you find the right one they sound pretty cool.
i dont know much about the turbo end of these cars but a bov should be able to be used on them, you dont "need" one but it wont hurt to have one, you should benefit from it. even if its just the cool "psssshhh" sound you can go cheapo off ebay but i wouldnt, the only one ive used on my other cars (tbird, boosted volvo) was the greddy type rs. sounds sick IMO
EDIT** you can still use one though right guys?