Congrats on the TS, you'll enjoy it. We put WRX wheels on our '03, and this improved the handling and looks over stock. BUT... the rolling circumference is a few inches larger, so if you go this route you're changing the gearing and speedometer / odometer slightly... only about 2%, but it is some difference.
TS 15" = 195/60/15, WRX 16" = 205/55/16, IIRC. A 50 series tire on the WRX wheel would be closer to TS stock.