The A57 is supposed to deliver 3x the performance of a15. Of course that means it would still need more cores than Xeon Phi. Interconnecting the cores also takes energy. I think it remains to be seen which is better in terms of performance/W.
Energy-efficient does not mean low power. It means a better quotient of MIPS or GFLOPS per power unit used. Those numbers are published if the processors in question ran LINPACK or for I/O, a similar test.