I'm an EE that did power optimization at Qualcomm a while back. I'll just say - if only it were that easy ! If everything lines up, yes, it's possible to drop power draw to ideal. But clearly the software (legacy vs new kernel) heavily influences actual physical temperatures on the same silicon/processor. So safe to assume the board (design, layout, components etc) is just about acceptable for this price point, even if not optimal.
Without a specific deep-dive into this issue, our efforts here are like band-aids the minimize the thermal performance problem with 4.x kernels rather than solve the problem at the root cause itself. IMHO, these are all 'toys' so I don't see anyone 'caring' enough to root-cause this (vs say, if this was my product line). Tinker on!