This was explained in the AMD thread, I posted a link to some really nice info regarding the impact of threads on DX11, with nvidia CMDLIST, but given there was no reply to that post I think no one read it.
Under normal circumstances HTT only really helps when a core is under very heavy load like maxed out so e.g. rendering tasks like cinebench. When stuff is natively multithreaded and does "not" max out cpu cores I would expect the impact of HTT to be very low.
So older i7 cpus hanging on more than i5 would be for two reasons in my opinion.
Older gen cpus are getting maxed out in some newer games now. So in those situations the benefit of HTT will increase, it wont be the same as in cinebench where the core is really pegged, but it would be higher benefit than moderate load.
Second reason is nvidia CMDLIST. On DX11 draw calls natively are single threaded, and you have a problem of the main thread getting too much work to do, and unless the per core performance is high enough to avoid a bottleneck then things slow down (AMD DX11 drivers). Nvidia added a hack that slices up the draw call load evenly across hardware threads, so if you have 8 threads you have 1/8th of draw calls on main thread, if you have 4 hardware threads youy have 1/4 of draw calls on main thread, even if the other threads have lots of spare capacity it doesnt matter, it just slices up based on hardware thread count. So basically you can eek out a bit more performance with HTT in that situation "IF" the main thread is saturated.
Given DX12 and vulkan should spread load across cpu threads more efficiently, then the main thread issue should be less of an issue and it should reduce impact of HTT noticeably, but I could be wrong, but thats what I would expect providing the cpu itself is "not" maxed out, once it is maxed out then reason #1 comes into play again. It is also why AMD have seen big gains on DX12.
8600k, 2600x and 8700k are in that kind of middle ground where they not 4 core or 8 core but 6 core chips. I think as far as 8700k and 2600X go they both have nothing to worry about, 6 cores 12 threads, the only concern will be if the per core performance on a 2600X is good enough. The 8600k has no HTT so is limited to 6 hardware threads, this from tests I have seen is enough to not have it see the issues that 4 core i5's have had. But for sure it will be a possible issue going forward, after that I learned about CMDLIST and how I have seen cpu utilisation creep on games, as a 8600k owner I have some concern but I think I will be fine for at least a few years and thats as an 8600k owner with no HTT, a 8700k owner with HTT I think will be absolutely fine.
I think we have got to the point now tho where intel need to scrap their HTT tax, and just have all i5's and i7's with HTT as standard like AMD do with their stuff.