Id set it for the tightest spring tension. New WRX dont like venting BOVs. Massive overfuelling on throttle lift off causes the backfiring.
A tighter spring tension will mean slightly less air will pass through the
bov, but you will still get over fueling and your overall fuel economy will drop also.
I have a GFB hybrid. Used to have it on 50/50 then full atmo, but now full plumb back.
My fuel economy has already improved significantly since going back to full plumb back.