^^Not to mention that the stock rims are also lighter. A WRX is not a car to go and by cheapo rims for.
Don't expect to find rims in any local shop for a WRX.
Anyway, to add to what InDeForest said, you CAN get away with a 48 mm...even a 45 mm offset.....if you have a rim width of 7.5" and above. Also, the type of calipers on the car will affect what type of offset you can get away with. If your car carries the regular sliding caliper brakes, they require less clearance, than say the 4 pot/2 pot brakes found on a few WRX's.
But as 'forest rightly said, you'll be changing the geometry of the car....not a good idea. Keep in mind that a car like a WRX, STi, Evo, runs negative camber on all four wheels (significant -ve camber in the rear). Heavy "cheapo" rims only reduce the handling capability of the car.
Plus using rims with different offsets can affect the handling....positively or negatively.
Here are some rims to consider:
5-Zigen FN01R-C
Enkei RPF-01
Enkei NT03