Originally posted by VoiceOfReason Wouldn't it still mess up registration distance?
Depends on the optical design. If the "lens turbo" telecompressor has the same K-mount to E-mount distance as a non-optical adapter, then it wouldn't make a difference. The light is just getting "concentrated" into a smaller image circle at the same image plane. If it's optically designed to focus the adapted lens at a different plane (but not sure why that would be done - that's just added complexity), then it could be less or more.
Unlike a teleconverter, these are not optically enlarging the image circle, but compressing it. Much easier to do within the original flange distance, I think... where's that optical designer when you need him/her? (I keep thinking of how the image gets larger when you move a projector away from a screen, etc.)