Having a 98 sti myself i can relate to this.
Your best bet as others have said is a used apexi power fc + commander
you can pick these up for arround 400 /450 and it will sell again for the same money if you ever decide to sell up. ,
there are no problems on mine running at a little over 300bhp + 1.4bar (vf24) , standard other than a decat straight through + pannel filter
+ the commander is a nice point of refrence for most stuff including a knock sensor display wich is priceless when tunning your impreza + it looks neat on your dash
