OK, so I was curious about this so here's what I've found so far...
I measured my T-adapter for EOS...the clear aperture is 42mm...which coincidentally appears a lot in Celestron's white paper for Edge HD
Then I did some basic calculations... the 5D Mk III has a chip size of 36x24mm, which gives a diagonal of 43.26mm. So from that, I'd guess you might see a little vignetting in the corners
It might be a little worse than that because any scope->T or powermate->T has to have some additional incursion into that diameter, even if it's thinly constructed. I'm no optical path expert but from my basic understanding you might see some vignetting when you try to utilise the absolute full frame, but it'd be interesting to see by how much.