I have the gearboxman T5 in my car. Didn't have any troubles with it with 400bhp.
My brother took it to the ring last year and he said that on the end of the lap, the 4 to 3 gear change didn't go so smooth anymore. But after a few minutes was fine again, so maybe it is caused by heat problems?

We'll see how well it goes with my new 550bhp (hopefully) new engine, which is almost ready