may be rubbish but newtiburon members claim that
headers
catback
c.a.i
ngm stage iv intake manifold (upper and lower)
intake spacer
performance leads, plugs and grounding system
ecu tune/remap
sees 220-225bhp at the wheels (with average 15% transmission loss for a fwd car that equates to 260-265bhp at the flywheel... for the cost of maybe £2k if you know what youre doing.
also you can get headers for about £99 plus import (this will be the stinger) from american ebay, still alot cheaper than buying new in uk, and despite brand names theres VERY little difference in them. i bought a set on uk ebay new for 320 and sold them straight on to faznloz at a loss because i changed my mind as with all the long distance driving the noise of just the motoria catback is getting annoying lol.
on another note, a supercharger convertion costs about £2k if im not mistaken, with fitting of course, and ive heard on a stock engine the supercharger achieves around 230bhp at the flywheel... something to think about....
...and then there are extra "stages" to upragde the performance from the supercharger setup, along with the rest of the mods above, not including the intake manifold as the supercharger replaces the intake manifold. with the supercharger fully upgraded and the rest of the NASP mods you could have a very impressive setup. there is a video on youtube of someone in a supercharged v6 coupe filming themselves slaughtering an impreza down a motorway (possibly american so search for "tiburon" instead of "coupe") to give you some food for thought.
however as mentioned the v6 coupe was only really designed to be a cruiser, and to compete directly only with the v6 ford cougar (which it did and it drove the cougar to extinction) not to be a sports racer, hence the insurance group 14, and why its so smooth on the motorways.
also to suggest why you lost the "race"::
the v6 coupe has 165bhp and hits 0-60 in 7.9s top speed 137mph
the bmw 120b ranges from 160-174 bhp and 0-60 varies from 7.3-7.9s, top speed 142mph, and has a lot more torque than the v6 coupe!
dont assume diesels are slow. id only expect to beat a 120d for the first 10metres or so then it will keep on pulling away into the distance, however you would give it a decent run round a track - WHERE I RECOMMEND YOU DO YOUR RACING FROM NOW ON!