Just going from first principles, it seems like a focal reducer could improve the MTF precisely because it reduces the size of the image---the same reason small enlargements look sharper than big enlargements. The extra element necessarily introduces some aberrations, but if the improvement due to the smaller image scale exceeds those aberrations, it could be a net win.

But I don't know how the performance of these things shakes out in the real world. Should we be fitting focal reducers to large-format lenses and mounting them on our 35mm cameras?