hi good to see another owner out there (although to be honest i'm happy it's rare to meet another owner in a sense).
my unfortunate personal experience is the gearbox is more likely to die from abuse rather than outright power. i've only got a 3" exhaust, front mount and a bit more boost and i killed my gearbox. i strongly recommend you take a look in the bluebirdattesa yahoo group as there has been a lot of discussion re gearboxes.
i've had no car for 2 months now and should be getting my gearbox back soon, been down south getting a ppg gearset installed (ouchie$). like a lot of people who've broken boxes i had to decide whether to just throw a 2nd hand one in, or go all out. i chose all out, because i decided that with a standard box i'd spend too much time worrying about it.
my biggest recommendation: shift slowly, and ensure is 100% in gear before getting on the power. i believe i shift a little quick sometimes, and when my gearbox went i was driving normal but kinda tired and believe i may not have put in gear 100%. i know two gtir owners who've had a similar thing happen (same gearbox more or less).
dont cry when you look at the gearbox pics
http://www.clubpoint.net/bluebird/