don't get a bov.. last dude was right... bad ju ju with rich shifts. Just get a sri (Short ram intake). WRX has many options available, but the best 4 that don't screw up the MAF are: APS Cobb K&N Typhoon and SPT.
APS Good, but ugly IMO, Works great with front mount intercoolers.
Cobb you hear stories about the high temp glue melting. Otherwise good reviews.
K&N I own, like it b/c of the heatshield
SPT also has heatshield, but you have to buy separate. Costs more than K&N but if you get subaru to install it, no warranty voidage.