With cams, header, pulley, intake, highflow cat, ProECM and a little here and there I was running with and beating most stock WRXs to 100-ish, figured about a 14.7 1/4 mi. The only way you're going to break into the 13's is to raise the effective compression of the engine and improve the breathing. Raising compression should be self explanitory... pistons. Breathing is both cams and heads (intake manifold and TB are quite good stock). You'll need to raise the redline to use the cams so now you're getting into valvetrain mods and either a unichip or full standalone ECU. At these power levels you'll probably have to close the block and maybe sleeve the cylinders. Then there's the clutch and tranny, stock tranny if babied should handle the power but if you tweak the gearing you'll be able to put down more/less power where you want it. Oh, then there is the weight... if you can loose a couple hundred lbs off the car many of the above mods won't necesarily be require but the car will likely not be street legal or safe (poly windows, CF everything, lightweight or no bumper beams, CF/FG bumper covers/fenders, FG/CF dash, you get the idea). Daunting huh?
This is why I went turbo