I really love the work Max Gabl did for TOS-R, the enhanced landscapes and matte paintings are my favorite improvement of the TOS-R release (except for the shuttlebay footage. Cropping the top and some at the bottom of the original VFX footage would have yielded the same effect).
Of course, the problem with the original TOS (live-action) footage - in contrast to the TNG footage - is that you don't have excess areas left and right of the "action safe" images to expand to a "full screen" 16:9 image (aspect ratio 1.78:1).
The overscan of old 4:3 TV tube sets would trim the areas around the "action safe" area. A flat screen (with traditional overscan active) would yield a 14:9 TOS image (familar postcard format) because no overscan crops the sides.
I think this is the widest TOS image we could realistically expect unless one were to crop off additional areas at the top and bottom which I wouldn't recommend.