Is it possible to have a 14nm 2/4/6 core die and a 10nm 8/10/12 core die?
Sure, but you're wasting a lot of time and money doing that. Optical shrinks are not free. And it would be a little weird if they did, let's say, 10c/20t Comet Lake followed by 6c/12t Not-Exactly-IceLake (14nm), followed up by I-don't-know-how-many-cores Real-IceLake (10nm) that had lower clocks but used a lot less power.
Yields are obviously the biggest issue; but clock speeds are a factor too. No point in moving the desktop to 10 nm if it's going to end up being slower in games due to the lower clocks, even with higher IPC.
At this point we have no idea what will be the frequency range or behavior of the 10nm process Intel finally decides to sell to the public. Remember that 7nm is supposed to be shipping late 2021 as well. 10nm is looking to have a very narrow window since the first products might not even ship for desktop until 2020.
The frequency that matters for gaming today is not the same as the max frequency the node allows for. (all-core boost vs. single-core boost)
Bear in mind that many of us overclock, and we'll be trying to hit that all-core turbo, all the time. Or higher. Trying, mind you.
Let's assume a pessimistic 10% drop in fmax and an even more pessimistic 5% IPC jump as far as gaming is concerned. Assuming core count parity, this pessimistic edition Sunny Cove could still run at 4.5Ghz all-core boost and match a 4.7Ghz CFL in performance. And that would probably be the 65W TDP chip.
I guess the real question is whether it could be overclocked to something higher than that? If it's anything like GF's 14nm then the answer is mostly "no" (it was stuck near the all-core turbo for the 1800x). If it's "yes" then we'll have to see how fast the power usage ramps up with the overclock.
The respected barriers for 14nm++ right now are in the range of 5.2-5.3 GHz for air/AiO depending on the chip (some are too hot for air). Intel doesn't actually ship a chip that runs those clocks on all cores all the time, but once we get our hands on them, the 9900k can do those speeds easily . . . with the right cooling. The 9700k even moreso.
You're welcome to make the case for 10nm not even being able to run at 4.5GHz, though. Where do you see it, back to 4.2Ghz as the original Skylake?
Er well, OCing factored in, Intel hasn't had a chip limited to those speeds since Broadwell (and some Broadwell could run faster, albeit at high voltages like 1.48v or higher).
Skylake was more like a 4.7GHz chip. Kaby was good for 4.8-5.0 GHz, and Coffeelake has gone as high as 5.2-5.3 GHz. Those are the speed limits we're really looking at right now.
I am sort of assuming that Comet Lake's many core turbo will be higher than Coffee Lake Refresh.
I would be really surprised by that. They already jacked up all-core turbo from Skylake->Kabylake->CoffeeLake->CoffeeLake Refresh. You think they can do it again on the same uarch with the same process? And in a 10c package? I doubt it. They're just gonna bolt on two more cores, TRY to maintain the same clocks they got with the 9900k, and hope for the best.
. . . so like Broadwell all over again? Let's hope not.