Zero loss of energy to heat in any of the conductors. You're not making the battery more efficient, you're making everything else more efficient.
Maybe if mag-lev cars are possible, you could get that kind of gain from the reduced friction.
That's no small deal, but in the grand scheme of things that a hot superconductor can give us.. I mean, this can (possibly, with decades of research) give us fusion, quantum computing, etc.
Agreed Fusion would be the biggest win possible.