I guess I was still pretty tired from getting up so early...so sorry about the details. It was the 1D X + 1.4x III + 12mm Extension Tube + 2x III + 300 f/2.8 IS II - in that order. The technical details are: f/9, 1/320s ISO6400 +1 &1/3 EV. It was somewhat windy and the tree he was in was shaded and moving, so not really a great subject because I had to use ISO 6400, which kills some detail on it's own. Manual Focusing wasn't too hard except that the owl was moving because the tree was moving. Perfect for AI-Servo, not so great for MF!
The CA was remarkably low, pretty much non-existent between the dark branches and bright sky, and just around 1 pixel wide around the owl's eyes. What I meant by percent reductions were in comparison to the bare lens in terms of contrast (ability to resolve differences between dark & bright areas) and resolution (ability to resolve fine detail). The sensor resolution stayed the same, obviously, and the magnification increased, but not enough to overcome the additional softness, at least in my estimation. Subjectively and based on this and a couple of other trials, I would say it's like this:
300 f/2.8 IS II: Contrast: 100% Sharpness: 100%
300 f/2.8 IS II + 1.4x III: Contrast: 95% Sharpness: 95%
300 f/2.8 IS II + 2x III: Contrast: 90% Sharpness: 80%
300 f/2.8 IS II + 1.4x III + 12mm Extension Tube + 2x III: Contrast: 75% Sharpness: 50%
Maybe I'll get some test charts out and compare under controlled conditions someday, specifically to see if up-resing the 600mm files is better than the stacked extenders (I suspect it is).
It's no substitute whatsoever for a 600+1.4x or 800mm lens, but an interesting tool if you'd like to get closer to a subject and don't need super sharp files. I wouldn't use it for paid work or until I had a nice shot with the 2x, but it's fun to play with if you have the opportunity.
The necessary high ISO made this test tough, but I'll try it again if a similar situation comes up under better lighting conditions, and I'll post those results.