To be honest, I would not bother about boosting it up to 14.5psi or 16psi or even buying an eBC unless you are having boost control issues. Save the money and spend it on a tune. Do it properly the first time. Running the car with a full exhaust system untuned will not ruin the car. You only run the chance of overboost on extremely cold days or high load situations and that's only if you boot the daylights out of it in these conditions, which would throw a CEL (which goes away after 5 starts in the GD Impreza). Normal driving, no problems. Spend the money elsewhere.

If you want BFYB modding, save the money and get a GD STI TMIC, VF series turbo, STI 550cc injectors, fuel pump and tune. Guaranteed trouble-free motoring with 180-200awkW. Best part, all these parts bolt on.

That's my 2c worth.