For more power, which the engine and gearbox can handle, a new transfer box might be needed.
AWo
Agreed. If power numbers increase they might have to go to a chain-driven design. There's just so much sheer force in a gear drive box with the consequence of excess heat. I can't see Ineos trying to make the current Tremec unit any bigger but maybe a larger dedicated radiator will suffice. Eliminating the center locking feature and relying on traction control will help too.
I'm not sure what the transmissions are really capable of. The B57 diesel models don't make all that much more torque than the petrol version and use the 8HP76. Perhaps there's not as much headroom to the 8HP51 as we'd like to think? If Ineos starts using the upcoming hybrid BMW engines and HP/Torque numbers jump substantially we'll know the answer by which transmission goes in behind them. Personally I wonder if the 8HP51 is marginal and was paired to the petrol model mainly for fuel economy. Lighter weight, less rotating mass, etc. Probably cheaper, too.