You know enough about CPUs to understand that Broadwell WILL have higher TDP. Disable 256-bit AVX and look at tdp then. Going to bet it will be 95W too, with Turbo clock too, making the clocks between 1800X and 6900k virtually identical.
So, 4.2Ghz it is?
I think that Zen will OC more than 4.3GHz, but OC will be useless, because of XFR. Just set TDP limit at 150-160W or higher (and an adeguate cooler) and all core turbo probabily will exceed 4GHz, with peak of 4.5-4.6GHz in single thread loads...
EDIT: for the first part... Mean blender clock of 6900K was 3.5GHz and blender is not an heavy AVX2 load. An overvolted Zen managed to draw less than stock 6900K. SO yes: i think that Zen will overclock higher than 6900K...