"The other angle, I suspect, is one of the side-channel attacks that can occur when HyperThreading is in action. By disabling HyperThreading on the volume production chips, this security issue is no longer present."
I find this argument quite odd, since I think most laptops are still HT enabled, all Xeons pretty much are HT enabled...
it's imo pure and simple the way to segment/limit performance to explore more price points and position against AMD/their older products and absolutely nothing else.
in any case, 9900K looks like crtl+c, crtl+v a couple of cores! it will probably still work on Z170 even with DDR3 with the right hacks!?