@Ænimal great post sir! I have to 2nd everything mentioned there re: tuning and the importance of it. The OTS maps, while keeping your car relatively safe, are not the answer in any situation. I have to admit, I was fortunate for the time I had mine, it worked alright. However, now with a pro-tune, the driveability has gone through the roof and I would recommend getting a pro-tune to anyone.
Re: the tranny and diffs on the STI, the GD STI tranny is a beast! If anyone ever needs to change a clutch in there, please, please, for your own safety have a tranny jack and a hoist.
Re: clutch changes being that I mentioned that, I would completely recommend the Exedy Stage 2 for any application. With or without the HD pressure plate, the feedback on engagement is much more positive than the organic disc. Speaking from experience going from stock to stage 1 with the HD pressure plate, there was not a lot of feedback as to when the clutch was actually engaging, now with the stg 2 and hd plate, no chatter, but you know the moment that the clutch starts to grip the flywheel.