Transmission failure is usually through fatigue rather than instant failure. You can put more torque through it but don’t expect it to last the 100,000+ mile design life that manufacturers work to. I suspect most tuners are not considering that sort of life cycle.The discussion of the current manual gearbox already being at it's limit in the Evora GT with 416hp keeps coming up, but where is the source of that information? Komo-tec clearly offers +70hp/+66lbft upgrades on the Evora without any modifications to the transmission.
I'm beginning to think it was a myth promulgated to convince people that more power isn't feasible in the Evora/Emira 6spd platform.
I have seen performance upgrades for other vehicles where I know for a fact that the transmission will not last long term but it doesn’t stop people selling it.
If you are only expecting to do 20,000 miles in your car before selling it you should be ok. For the person who buys it off you, they may not be so lucky.
Can anyone think of a manual gearbox in a transverse engine installation rated to more than 420Nm? The highest I can think of is the Focus RS at 470Nm.