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.
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.
What are the engineering and design challenges in creating successful IoT devices? These devices are usually small, resource-constrained electronics designed to sense, collect, send, and/or interpret data. Some of the devices need to be smart enough to act upon data in real time, 24/7. Specifically the guests will discuss sensors, security, and lessons from IoT deployments.