Thanks for doing the math I was too lazy to do....
So, it does indeed sound like a eInk display of the same calibre as Kindle's latest/greatest is feasible.
I'm not sure if Kindle's latest offering is in fact the best eInk tech out there...

But assuming bandwidth requirements would be quite similar for the best offering.
It seems we could (theoretically) have a "world's best" eInk display available as a "Other Half".
This could be quite the marketing coup, if done well....

Personally I prefer my eReader to be a discrete device & about 7", but I'm sure others would like this approach.