I killed 3rd gear on a standard gearset when I was running 800lbft+.
I now have a treated gearset and have limited torque to circa 800lbft to keep it happy, but am running close to 1000bhp. 2 events and circa 1000 miles later, no problem. But that's still hardly a test of longevity.
I will be upgrading to an aftermarket gearset asap. As it's the only way to run a lot higher power/torque and keep peace of mind.