The M8 and Gs5 have better battery life than their predecessors, and are among the best non-phablet phones in terms of battery life. The Anandtech M8 review in particular points out the generational leap in battery life from chips of a year ago, and how it extends even beyond the battery life gains of the 800 chip. But, let's suppose for a second that the 801 was ONLY a clock boost and a slightly improved process.........SO WHAT?!? I still want that on my next phone rather than last year's chip. Why is that so hard to understand?
Why do we even put up with that crap! First, it was the CE architecture that held us to single core chips, then WP8 allowed for dual core and quad core, and I don't understand why the architecture won't support whatever chip the OEM wants. The argument used to be that the chips were specifically optimized for the OS, but my wife's 1520 running on a 800 is not significantly faster than the 400 on my 1020, so whatever optimizations are there are horsecrap. Those same gains can be found if you go from the Android phones running SD400's vs. the ones running SD800's and the OS just supported it. The OEM's didn't have to wait for the OS to be "optimized" for those chips.
The predecessors to the M8 and SGS5 ran on the Snapdragon 600, not the 800. Looking at your own referenced Anandtech M8 review, the M8 (801) battery life is negligibly better than the LG G2 (800)
I understand why you want the latest, i'm just pointing out that it shouldn't be a deal breaker in this case because the difference really is negligible this time. You should consider the end result instead of just wanting the latest and greatest.
Having not used an Icon or 1520 I can't comment on the perceived improvements in performance, I will say that the benchmarks blow the previous generation out of the water though.
I'm not sure what Microsoft has to do for each SoC, but it sounds like the difference between an open platform vs a closed platform. You can't expect a closed platform to run on whatever you want to throw at it.