We traveled 2-up on a F800GS across S. America, from Colombia to Ushuaia. 5 months, 27.000 klms.I'm also 175cm and 70 kgs.
It's a perfectly capable bike to go wherever you want. Really, there is not much difference with the F650/F700, maybe on some off road stretches the 21'' front wheel of the 800 will give you a better handling.
Taking into consideration my height and weight I wouldn't choose the 12GS. In fact, if I could choose now, I would take an even lighter bike than the 800.