A medium (or even low) resolution photo is sufficient to create the morph geometry; which was my main reason for looking at Face Shop in the first place.
I think it is unreasonable to expect a single medium or low resolution photograph to produce a high quality 3D representation; however, the face geometry is there, and there is no reason that you can't use of a different and much higher resolution texture on said geometry.
Face Shop creates Unwrapped UV co-ords. You can extract a template; or just unwrap it again, if you don't like the existing Face Shop UVs.
Then there is no reason to expect the new morphed model and textures to behave any differently than any other model.
EDIT - some of the pictures look rather good right out of the box, which surprising to me. I would not have expected a single picture to provide enough information to make a 3d likeness.

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