I don't know any photographers who shoot handholding with super telephotos that would not applaud losing almost 2 lbs of camera weight. I shoot BIF with a 1DX2 and a Canon 600mm f4 lll. Doable but a work-out!
With its lower pricing, performance and features, the A92 is taking a share of the enthusiast as well as pro shooters. I just hope the 1DX3 offers superior AF. We will see soon.
It's not the loss of weight overall, which I agree is a good thing, but how the weight is distributed. Mind you, the latest supertelephoto lenses are also a fair bit lighter, so the balance may still be fine with a lighter body.
Incidentally, bravo! I have tried BIF with the 500L II and found it too cumbersome to do for very long (mind you this was swifts, maybe it's more feasible with bigger birds). I figured using a shorter focal length with a much lighter lens was the best compromise (in that case the 70-200 + extender was what I had to hand).
Edit - I missed that you said the mark III. Even so, impressive! How easy do you find keeping moving birds in the frame at such a narrow FOV?