With a full decat exhaust on a MY01 WRX and on, with an ITG air mat, we have been able to dyno results at over 260 BHP. In order to get more BHP & Lb/Ft torque, other adjustments need to be made, but not bad without any engine work!!
I would not advise the superchip route, will not comment why though. However to really get extra power out now, a replacement or piggy back ECU will be required, one that fully remaps, not one that adjusts the boost control. An ECU is being worked on that will boost power to over 300BHP with the above-mentioned mods.
Any more than that, you will need to rework the engine i.e. head work, bigger turbo, equal length manifolds etc.
The MY01 and on is a heavier car, app 130kgs heavier, but it is a better car and faster to drive on the twisties/circuit compared to a comparably modded pre MY01 Impreza (we have proved this many times), because its stiffer by a big margin and this makes a big difference.
The 4 pot brakes are OKish but are not up to the job of proper driving/tack work (they fade too much!!)
At the end of the day, its up to you but its not always about how quick a car is in a straight line (unless you are of the traffic light boy racer fraternity, which I am sure youre not.
Help the engine to breathe better (£500-700ish) and youll be on your way. The standard set up will just about handle that power increase but id advise improving the brakes and then the handling and im sure youll be happy. Itll still be cheaper than an STI also.