the thing is, doing these launch-controlled starts is so much harder on the transmission than any race track driving will expose it to (super high oil temps not withstanding).
just think about the physics here- it's a 1740kg car, with wiiiiide&grippy tyres (140 treadwear rating from the factory), a super-fast response AWD system that almost instantly goes to 50:50 torque split, and an engine that makes at least 580Nm of torque at launch revs. doesn't take a real braniac to see where this is heading...
i think the problem is thus: thinking back to high-school engineering science, metals have whats called 'plastic' and 'elastic' strain limits. the 'elastic' limit is how har it can deform and return to the original shape (think of flexing a metal ruler gently), whereas the 'plastic' limit is how far the object can be deformed before it fails catastrophically; though it won't return to the original shape after the 'elastic' limit threshold is exceeded (again, think bending a ruler until a permanent crease forms)
what i think is happening with these R35's that have had gearbox failures, is that the owners that spend all day smoking everyones arse off at the traffic-light GP are taking certain teeth on the 1st gear gogs up to and past the elastic limit with every launch. eventually most, if not all, the teeth on 1st become deformed past the elastic limit, and the next time the car is launched with the cogs in the fatally weakend spot, it just lets go.
that's my theory anyway- if it wasn't the case, it'd just die the very first time you tried it.
and looking at the shots of the busted box, they really aren't small gear faces, either...