The question is not
whether or not it will manage this kind of trip - surely it will as its a very proven bike - but how comfortable you and your pillion will be, and what are your own personal views of a bike being "enough" for something. That may vary significantly from person to person.
Surely the new F800GS is much more powerful, modern and probably better handling than a Transalp, which is a way old construction. But the Beemer's lot more costly as well. Personally, I would wait and see to make sure they havent released a 'beta-version' this time, like they seem to have done with their newest-generation boxer.
If you decide to change, you dont necessarily have to go bigger - the Suzuki DL650 is also more modern, and on another level from the TA regarding power and handling. It can even be loaded very heavily, and still do the job alright. I've just tested this on a 16000km trip from Europe to India two-up, and we have luggage like you wouldnt believe, even a set of extra tyres hanging on sides of fuel tank!! Its worked like a clock in any condition, it really is tougher than it looks. And the DL can be bought quite cheap, especially second-hand.