Why is it so hard to believe that there is upper limit of torque a transmission a can handle. 850 Nm is 35 percent more than 630 Nm and that is a big number. There is a good reason why AMG tends to use old 5-speed gearbox for its 65 models that develop 1000Nm and not 7G tronic.
But still,I agree that transmissions are often much stronger and thats proven in real world tuning.-DA Motorsport 1000HP BMW E36 M3.
Go to ZF or Getrag sites and you'll see that every transmission has max. torque capacity number,you can put additional load,and it might stand it,but durability and therefore reliability comes into question. For instance there is no way Porsche 997 turbo and HGP R32 will retain the same level of durability of most components and not just tranmission.
At the end i'd like to say that I strongly believe that this MODDED transmission that Brabus is using is very well built and strong,Brabus has proven numerous times that their cars are sometimes even tougher and more durable than MB itself.Its truly one of the worlds very best tuners,out of the question.