There is a very interesting article regards 'Human Eyes' that has done research as such... and much is told via the eye. It is of course a mirror to the sole. -Likewise with horses etc.
The 'Eyesign Theory' will always be just that till the so called experts can stop quibbling, trying to impress, and get down to some fundemental, realistic basics.
I like the eyes of my pigeons to be a pair, and preferable one each side of it's head.
Of course there is, and always will be many claims made, and all unprovable.
I mean just where would the money come from to research it sientifically... let alone a worthwhile reason for it, that can merit the time, and costs.
So in that reality of course one must draw largely upon their own expierences, results and what has been notted.
One other fact is that there are more good 'Eyes' than good pigeons, ... But then again, it may well be as the great peerless Bob Flemming said. 'Have seen many a good eye on a bad pigeon, but never a bad eye on a good pigeon'.