It is all clear to me from this chart (which was part of an Intel presentation this spring).
https://en.wikichip.org/wiki/File:intel_14nm++.png
The red curve is Skylake that the Coffee Lake is basically based upon. The orange curve is Coffee Lake. The average power loss per fin is about 52% lower on Coffee Lake compared to Skylake (assuming you are going for the same performance).
Now for some very crude math (yes the math could be done better).
- Skylake 6700K: 91 W / 4.0 GHz max turbo / 4 max cores = 5.688 W/GHz/Core
- Coffee Lake 8700K rumor: 95 W / 4.3 max turbo / 6 max cores = 3.682 W/GHz/Core
Now what is the difference in power between those two numbers? 5.688 / 3.682 = 1.54. The Skylake 6700K uses 54% more power per max clock, which matches up very closely with the 52% power savings of the 14nm++ process.
Of course that math was with TDP and not actual power used, and it isn't take into account many fine details (more or less thermal throttling, a chip doesn't have to stay on the same performance point on the curve and can get more or less power that way, software capability of keeping 12 threads active, etc). But it shows you roughly how it can be done.