See, it all depends on how you drive, where you drive, and how much weight is in the car.If you only drove on the stock setup for a week - you don't have a valid measurement to compare against.
That said... 21 seems low for the WRX. That is what I get for my STI.
Demon has the answer. The trade-out-and-up option is the way to go.I have run Falken in the past with no issues.....that being said...
Since you have been having troubles with them, and they should be wider, and they will give you a full credit to change them out.....I would change them, if it were me.