I'm finding it difficult to Google how efficiency scales with magnetic flux. I've ended up at a thirty five page pdf https://arxiv.org/pdf/1409.3540. Equation 1 on page 5 states the fourth power dependency. So using REBCO magnets, instead of Niobium-Tin, to get twice the magnetic field looks like a really big deal.

Page 6 adds a twist to the story. "For REBCO superconductors operating far below their critical temperatures, the toroidal field is generally limited by mechancical stress rather than the critical current density, which typically limits standard Nb3Sn." So further increases in magnetic field might be waiting on advances in mechanical engineering design, not on better superconductors.