All drivetrain components are rated in torque not bhp, the common factor for gearbox is 2.5-3times oem torque value. This is to ensure against destruction from standing starts, fast gearchanges etc where the instantaneous torque factor is higher.
This means you can run more power as standard but as soon as you use that power for faster gear changes standing starts etc this is when it will break.
This also accounts for driveshafts, hubs, anything where you get that initial shunt of torque when engauging the power. The person running close to 300bhp has had major driveshaft issues & according to the regs the Trofeo cars must change the hubs every 600klms!!!though a lot of that is down to failures with the extra grip & therefore forces extracted on the hubs.
It has been said a few times that the 5 speed gearbox is on the cusp of it's limits in terms of torque hence why the Trofeo cars use the stronger 6 speed box from the Punto as TBH the Trofeo cars don't need 6ratios as many drivers commented on the ratios being too closely stacked.
As with any upgrade to a car it can expose weaknesses elsewhere.