Must-read op-ed that lays out the arguments:
In discussing the morality of a single-payer system, those efficiency considerations are irrelevant. In discussing the morality, one thing matters1: who is made better off, and who worse off, by the system?
Most advocates of single payer, I think, care most about this justice claim. They may also think that they can make the system more efficient, but if one could somehow prove scientifically that a private system would be cheaper and better, they would still favor a public system as long as a substantial population remained uninsured.
But wholesale transfers to large classes, from large classes, are not good moral philosophy unless those classes are very well specified to the moral effect you are trying to achieve.