can a bov like a synapse or hks actually improve performance on a 02 wrx?

Blow off valves make no gain in performance what so ever. However during throttle close under boost all that pressure builds up behind the throttle butterfly and that can lead to intercooler piping and tubing clamps to blow off.

l have the GReddy ssq bov on my 2002 WRX & have had it on for almost 4 years. l have SCCA raced it on the weekends & have put about 40k since the install. Personal Opinion: Yes, it improves performance, but by a margin that u will not be able to feel. The GReddy system has a hard/soft manual adjustment on the front to change spring pressure behind the valve. As the boost pressure builds the valve can can release sooner (7psi) or later (20 psi) depending on what u r doing with it. This can save ur engine from over boost by bleeding boost pressure off before it gets shoved in the engine. Or u can loose boost by mis-adjusting it.
Most people do not understand the principles of a BOV & install it open atmosphere without any ECU adjustment, & then tune the adjustable proportioning valve until it ''sounds cool''. This causes the car to run rich, trips the check engine light, puts black carbon on the bumper, & can cause serious damage in the long run.
l will always take on a race with someone that has a black cloud on their bumper, they obviously do not know how to tune right.

P.S. It will not make u faster though.