I have a GSA and love it, but think that you would be nuts to take one on to the pamirs. I also think that the 800 is too big.
Frankly, I don't see why you would need anything more than a 650. As to the Europe leg, there are plenty of wonderful small roads where anything more than a 650 would be a waste, and if you are planning to ride though Europe via expressways I would rethink that part of your trip and take a smaller bike.
I have a GSA (ridden through China, Russia, Morocco, etc.) and a BMW G650X XChallenge (ridden through Siberia, Road of Bones, etc.) so have actually done both.
Obviously, everyone is different but that is my two cents.
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