Co-processors don't matter for power consumption if they aren't being used because you can always just turn off the power to those areas of the chip. People talk about a coming area of "dark silicon" where transistors keep getting cheaper without getting more efficient. That means we'll keep adding more and more specialized structures to chips which are very fast and power efficient when in use compared to a general purpose CPU but each sits there dark and silent most of the time.

Back in the day the advantages of RISC were that since everybody had small teams of designers you could spend less time implementing instructions and more time optimizing and adding features like pipelining. And also that RISC, when introduced, could be squeezed onto a single chip whereas CISCs needed multiple chips which resulted in big speed penalties. These days everybody has humongous teams and much better tooling and more transistors than you can shake a stick at. The complex decoding of x86 instructions gives you a 5-10% power penalty on big cores (and a larger one on small cores). And also the strickter memory ordering requirements of x86 might be a disadvantage, I've heard contradictory things. But in general the performance differences between ISAs is pretty small at the moment.