I hike quite often with my 5d2 and 3 Zuiko primes ( 21, 24, 28 ) and the weight savings is very noticeable. I still want the image quality from my 5d2, but don't need the bulk or weight of full sized lenses.
I don't understand what you are arguing about. Size and weight are not a concern during a 20km trek with elevation gains of 1500 meters?
Your answer - xpro-1 or OM-D. Lugging a 5D2 with a pancake is like removing the windscreen on a truck to save weight.