well, I don't shoot with a 5D, but have multiple 50D's, the 1Dx is a bit heavier, but I'm used to shooting with a 70-200 f2.8 and sometimes a 2x extender, so am conditioned for lots O lifting anyway LOL
that said, I'm not sore after a day of shooting. I don't "hang" the camera, just hold it in my right hand, rest it on my lap, or cuddle it in my left elbow, if walking ... so no neck pain involved ... when I shoot fashion shows, I may hang it, but don't think it is enough of a difference to kill me, as then, I shoot with two bodies & lenses both hanging
and, to shoot at ISO 40,000 that is fourty thousand! I'll accept the additional weight