here's what's best for your car. Keeping it stock.

Once you start modding, it costs money. If you aren't prepared to fix what you break, then dont start messing with it.

The stock airbox is going to be fine for whatever it is you want to do to your car. If you ever plan on going more than 300-320whp, then you will need a better air delivery system. By that time, your WRX will be breaking and you'll need a new motor. Then, you'll go 2.5 and go rotated so it won't even matter at that point.