Upload & Sell: On
The interface on the 1DX is very similar to the 1DIII, 1DsIII, and 1DIV. They all use basically the same batteries, too. The 1DX has a slightly cranked-up version of the LP-E4 battery, called LP-E4N, but they can be mixed & matched with LP-E4 in cameras and on chargers.
I sold the 28-300L because I rarely used it. That's because I almost always have two cameras with me, with the lens combos I mentioned earlier. Even when I take one camera, I often bring my 24-105L and 100-400L. My "day bag" for local, non-photo related activities usually holds a 5DII, 24-105L, and 100-400L.
In my current situation, I use two and sometimes three bodies, and so there's no need to go to a "compromise" setup with one camera and a 28-300L. I say compromise mostly with respect to the range of focal lengths; 24mm to 400mm, vs. 28mm to 300mm. I find the difference between 24mm and 28mm is often meaningful, and 400mm always beats 300mm when you want more reach. Two more aspects of compromise associated with using the 28-300L, 100-400L, and 24-105L are (i) their IQ at the edges and in the corners of full frame images is not as good as the 24-70L II and 70-200/2.8L IS II, not to mention certain primes that are better again, and (ii) they're relatively slow in terms of aperture, which affects max. shutter speed, OOF behaviour (i.e. bokeh), and AF performance in low light. In many situations, these differences are not significant in terms of the resulting photos, but sometimes they are.
There's nothing wrong with the 28-300L, at all. If I ever get in the position that I need another one, then I'll get it. OTOH, sometimes the 24-70/2.8L II, 70-200/2.8L IS II, and 300/2.8L IS provide better results.