I would assume that they aren't going to stop selling tele-converters, so I wouldn't be surprised to see them release updated versions of the teles not too far down the road. I wonder if this is a Digic 5 issue?
The Kenko does't just hand the lens protocol through unchanged but seems to modify it a bit, for example to enable f8 af even on cameras that Canon doesn't want to have it. Canon obviously has changed the lens-camera communication a bit on the 6d/5d3, either to enable new features like dual afma or simply to torpedo 3rd party manufacturers. Once Kenko has reverse engineered it I'm sure they'll release an updated tc, however I'm wondering why it takes them so long - maybe they want to sell their current batches of "soon to be old" dgx first.
Imho all 3rd party products in this price range should have some mechanism to update the firmware for new cameras, tcs, flashes, lenses, ... I'd rather pay €50 for a trip to the service than €250 for a new tc.