[Scythels]

 

The one thing that concerns me when they make a single lens with multiple mounts for digital camera; is that there has been imperical proof that the rear element design needs to match the system the camera is used with for optimal performance

Much more goes into the placement of the exit pupil than the rear element.  Arbitrarily thick glass will more or less always work if the exit pupil is very far from the sensor.  The angle of incidence measured off of the normal of the rays hitting the edge of the sensor will be minimized if the exit pupil were at infinity (i.e a rear-side telecentric system).  Small AoIs mean better performance with thicker filter stacks on the sensors and more consistent performance across different stack thicknesses.  The array of microlenses on the sensor can only be optimized for one AoI which is likely a 'medium' one, but this will not especially hurt designs that have a distant exit pupil, as they should fall within the field (think collection area) for the lens they should hit. 

 

This is a telephoto lens, so the exit pupil is quite distant to begin with.  Distant enough that it wouldn't merit special consideration in the design and it likely 'just is' whatever the distance is. 

 

The lens would be optimized for canon EF mount, as this is the most popular mount and drives the most sales.  a 2mm thick coverglass would be used in the design of the lens.

 

 

Well, it cannot be much more compact or lighter.

Sure it can.  Aberrations with field dependence (coma, astigmatism, distortion, field curvature, and 5th order spherical) reach lower max values on a smaller sensor.  This would allow the pursuit of a greater telephoto ratio (=shorter) and the chief ray (touches corner of the sensor) defines the size of all elements after the peak in negative power of a lens.  For the new samyang, everything after the third element could be shrunk down.  A more tapered barrel could also reduce size.