[Galaxy S6 General] poweroutlet:
Technically the Core M will dissipate more than 4.5w for "bursty" workloads but under longer steady workloads it will average to 4.5w. The ARM tablet and phone SOCs more or less do the same thing. In terms of actual battery life test results, yes the battery life of most of these devices hasn't really changed since the last generation Intel U series chips but that isn't a real apples to apples comparison. As SOC power consumption continues to drop, it is becoming a smaller and smaller chunk of total system power consumption. Lenovo did a poor job IMO in originally implementing the first Core M device but Apple will almost certainly do a much better job. The SOC is only one part of the system, it is the responsibility of the OEM to properly package up the device and do proper power management, provide an adequate battery etc. Yes the new Macbook doesn't get significantly longer battery life but it also weighs only 2.0 lbs and has a ridiculously small battery. It also has a much higher resolution and more power hungry screen and yet manages to keep battery life equal with the last generation. Benchmarks have also indicated that the newer 14nm Intel CPUs are much better at sustained performance compared to the older 22nm Haswells. This is something that phone and tablets typically are very poor at.

A very fair point, browser benchmarks are especially notorious in being very misleading. I think in this case Chrome was used in all cases which helps a little. My point in showing this is that we need to take those GeekBench results with a little grain of salt. Outside of that benchmark, I don't think you'll find the A8X or Exynos 7420 getting anywhere near a higher speced Core M let alone a i5-5200U at any real world use or any other benchmark, browser based or not. Even other synthetic benchmarks like 3dmark Physics, etc don't show the Intel CPUs nearly as low as GeekBench does.

This is correct.

It is certainly possible for Samsung to have a design that is more power efficient than Intel when it comes to making a 2W phone SOC, but that won't be because Samsung uses ARM ISA while Intel uses x86. At this point, ISA is mostly just coincidental and isn't going to greatly impact the characteristics of your CPU. The CPU design and the ISA that the CPU uses are different things. The notion of "better design" is also a little tricky because a design that may be best for a low power SOC may not necessarily be the best for a higher wattage CPU. Intel absolutely rules the CPU landscape from 15w and up. Despite all of the hype around ARM based servers, Intel has continued to dominate servers and has actually continued to increase its lead in that space since Intel's performance per watt is completely unmatched in higher performance applications. Intel's big core design is just better for that application than any ARM based CPU's. It is important to remember that just because you have the best performance per watt 2 watt SOC, doesn't mean you can just scale that design into a beastly 90 watt CPU. If it were that easy, Intel would have probably easily downscaled their big core chips to dominate mobile SOCs.

You frequently find some people trying to reason that at 1.2 Ghz Apple's A8 SOC is very efficient and fast and then they claim that if they could clock that SOC at 3+ Ghz then it should be able to match an Intel Haswell core, but there is no guarantee that the design will allow such high clocks. You have to consider that maybe Apple made design choices to provide great IPC but that IPC came at the cost of limiting clock frequencies.