So with the Sony a7r, do the clipped corners affect the image in anyway? Surely not since no one has reported black corners. How does that work then? Is the entire sensor area not used then? That would mean in reality a sensor size slightly smaller than FF, prob negligible though overall?
If that's the case then perhaps a FF sensor could be squeezed into the M mount then with a tiny bit of cropping. Or Canon could make a completely new sensor that is almost FF in size and fits perfectly (seems unlikely in terms of maximizing profits).
The clipped corners aren't a problem because of the way light travels when leaving the lens - think of it as an expanding cone, so the image circle is physically smaller at the lens mount than when it hits the sensor. Just look at the back of the EF 40mm f/2.8, the rear element is significantly smaller than a FF sensor:
OTOH, there may be issues with some lenses with a large aperture and an exit pupil close to the image plane (similar to the 'clipped' bokeh of the 85L wide open with close subjects).