V5/6 STI boxes are on there limits at 400ftlbs, and probably on borowed time if being used hard, i mean they will last for ever if doddling around the place at 400ftlbs, but track, drag, or hards use its gonna end the one way.
It was never really an issue untill 2.5s were started to be used, and straight away the torque jumped on these cars, you may well only have 380bhp, but its quite possible to have 400or more ftlbs on a 2.5 engine.
As for justifying it, well its a horses for courses thing isnt it, after all how can any of us justify spending the money on fuel, tax, insurance etc etc.. but we do. So its really up to the owner, you will never see the money back by selling the car complete, and with newage STI gone so cheap it ads another dilema
There is 2 choices when it comes to increaseing power, you can leave the 5spd in and wait for it to break and then go 6spd route... or remove 5spd now when its in perfect working order, sell it and off set the cost of a 6spd.. thats the way i went about it, and iirc think i was able to get 800e to go towards the 6spd.