Assuming that a force ghost projects an image of themselves as a "whole" being in the force (ie Anakin without his scars, Ben and Yoda at their most powerful as old sages) wouldn't it follow that Luke's force ghost would project the face BEFORE Mark Hamill's face was damaged?
Just a geek thought... I think I may have found a reason to buy another Yavin Luke.... Hmmmmm....
I believe it was stated when Hayden was put into ROTJ that the reason was the Force spirits reflect your final image of good, and Anakin wasn't good long before dying in the Vader suit. Had GL really put effort towards putting Hayden in, he would have had him in full ROTS garbs, but instead they superimposed his head on Shaw and it looks off because Anakin never wore those clothes. Even if it was Shaw it would look odd now that we know what Anakin used to wear, but back in the OT days, I, and many others I'm sure, assumed Old Ben and Anakin's garbs were the full Jedi clothing, not retired Jedi recluse garbs.