yeah, the stock subaru air filter is good for like, a bazillion miles. (50-100k). It also is not very restrictive. There's no reason to change it on a stock turbo.
Any drop in filter won't mess anything up though, they all just don't do much different. Many are washable so they have unlimited miledge but then again they cost more than the stock one so it would take quite a few go around to pay for itself.
The problem with many aftermarket intakes is they mess with the MAF sensor. Using a CAI often changes the diameter of the pipe the MAF reads from and creates more turbulent airflow which can throw it off. Again, there's nothing to be gained here performance wise. The stock unit can handle the stock turbo pretty well.
04 STi 16G powered lagless fun
04 WRX: SOLD