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Yea, I think that's generally true. With that said, I am kind of curious if one would be better off using the 5D + 1.4x + 500L II instead of the 7D + 500L II. Sure, you have to give up one ISO and about 1 mpx, but according to Dxomark, the 5d MKIII is close to 2 stops better in the noise department. I'm fairly sure stacked 1.4x's on the 5D would be inferior to the 7D + 1.4x + 500.
Also Chris what about AF speed? Will 5D III's superior AF be enough to compensate for slow down of AF when you attach a 1.4x TC? I'd still think the 7D + bare lens will be a bit better, but would need to verify this.
I agree if you need a 2x on an f/4 lens, the 7D + 1.4x + lens combo should be much better for AF than the 5D III
BTW for those wondering what the pixel advantage is of a 7D + lens vs 5D III + lens + 1.4x it's simple. Adding a 1.4x is the same as using a sensor that had 2x the number of pixels, so a 5D III + 1.4x would give me the same reach as a 44MP FF. If we crop this to 7D FOV we would get 44/1.6^2 = 17.2MP. So about a wash.