Re the weight issue (and FF), as I indicated in my original post, I'm using on 1DX - total weight 5.4kg, too heavy for my Manfrotto tripod and head which are rated at 5kg :-(
So yes, weight is an issue, but it's something I've found you just get used to. When I first handled a 7D I thought I'd struggle to manage it. Again when I first got my Sigma 120-400 I thought I'd never be able to handhold it, let alone pan/track racing cars with it - now it feels like a bit of a toy. (I got a lot of sharp shots at 400 on my 7D, but the 1DX seems to show up that lens' shortcomings.)
Shooting racing, I'm on my feet and carrying the lens round my neck for hours at a time. One issue that does annoy me is the form of the tripod foot - the edges are a bit square for comfort. I'm looking into a BushHawk as I think that would be ideal for tracking cars and also for carrying the combo around between corners, but can't find any agencies in NZ, and I've heard some folks had issues with the cable reliability.
Anyone with experience of 1DX + large lens on BushHawk?
Re backpack, I managed to fit it into my Kata 123 Go bag, admitted not attached to the body, so I lose the "slide, grab and shoot" convenience. I can fit my 70-200f4L IS and 24-105L and/or 100L Macro either side, with the 1DX in the top compartment.