Fuck trying to match an a7S and any Canon camera... Sounds awful.
Why stick with Canon at all? I mean, sure you have some gear already.. And they aren't exactly "bad" but there are so many better options right now.. Amd let's be honest, Canon aren't going to cannibalize their Cinema EOS line with a decent DSLR (or even mirrorless) any time soon.. It has never been part of the Canon philosophy.
My point is this.. It just seems silly to drop so much on a MkIII when both the GH4 and a7S are better and far cheaper....... You can also adapt your Canon glass to either of them if you wanted too.... Why not just get rid of the MkII and 7D buy a GH4 or a7S and adapt your Canon glass? You might even save some momey in the long run bh not having to drop so much on a MkIII that aren't even in the race any more.
Obviously try the GH4 and a7S before hand... Or wait for the a7S II or a9.. SONY release a new camera every month it seems.
Both companies also offer better firmware support/upgrades, Panasonic especially.