AlanF, thanks for making the effort to provide this feedback. Do you think AFMA is a factor here? I guess the camera wouldn't be able to recognize/remember the combination??
Do you feel the result is better than a crop of less than full stacked? Did you play with sharpening the stacked result?
Maybe you've answered this previously, but you're gaining more experience daily. So, assuming you need reach to frame a stationary subject, does the 70D outperform the 5D3 in good light in any regard, considering the various aspects of IQ?
The AF is so limited for the stack that I used manual focussing through the viewfinder so AFMA doesn't come into it.
When I got good focus, the results were very acceptable. Using a tripod for a static subject, for moon shots etc, I think the stacked TCs could give an advantage. I got a reasonable shot of a bittern in reeds at the very limits of resolution which was at least as good as just using the 2xTC. But, the whole rig is too cumbersome for my hand held shots, which is what I want to do. And the loss of AF is a killer.
The 70D does give better resolution than the 5DIII, with DxO noise reduction and a bit of PP. But, this comes into play when you are cropping so much the results are never going to be spectacular whichever camera you are using. I am very happy with both the 70D and 5DIII, but the 5DIII will be my workhorse. I expect to test the Tamron 150-600 the week after next. My guess is that it will perform significantly better on the 5DIII. Its sharpness is very similar to that of the 100-400, and that lens is much better on the FF than crop. You need a very sharp lens to get the best from crop sensor.