So you don't recommend the Kenko adaptor? Any reason for that? I feel like the Canon is over priced a bit. And for my purposes v2 should suffice right?
I have no experience with the Kenco adaptor, but from what I understand, the Canon is optically better and likely to carry on working well with all future (TC compatible) lenses, bodies and firmware updates that you might find yourself using in the future. That certainly can't be said of third party products, although due to the Kencos compromised optical design it doesn't have an element which pokes inside the rear of the lens, allowing it to physically mount to many Canon lenses which aren't designed to work with TC's.
Here's a comparison of the mk II and the mk III 1.4x TC with a 70-200/4 IS on FF:
The corners are improved with the mk III, everything else looks quite similar to me. As for the performance on the EOS M, unfortunately this tool doesn't have the mk II TC samples on crop. The extreme FF corners are a non-issue with the crop sensor, but the higher pixel density might reveal some differences not visible in this comparison.
Corner performance isn't a priority I'll be using the center more than likely. I guess the extra cost for the Canon extender over the Kenko is worth it if it is optically better.