ok where to start
first things first, for the most part quad channel vs dual channel is mostly a meme, the difference is there on paper and some work loads, but the speed is really only beneficial on tasks that a server would be better for.
now 8 core, 4 cores per die being slow? yes, in gaming it would likely shit itself with inter die latency, however most applications where you would be considering a high end desktop as an option over the consumer don't rely on inter core communication so much, these applications are the ones that show off scaling per core better then anything else.
now pcie lanes, if you are rendering 3d, this isn't a question, if cuda accelerates it 4 980's would be 8000 cores, and cost 5-600$
4 980ti's would be 10000 cores for 1000$
now to get even close to 8000 you would need 4 1070's, and does that even work? if it does, you need to spend 1200$ on it, and to break 10000 you would be spending over 2000$, now if you can't do more then 2 way, to even approach 8000 cores you would need to spend 2400$
Now let's say your workload needed high amounts of ram, which 3d rendering can demand, thread ripper would allow for a cheap solution for 128gb of ram
then you have storage, while I would personally wait till nvme got larger solutions before I get another, if you need the space now, you can use it now, then you raid 0 them all and you effectively have a near 10gbps read speed, which is orgasmic for video work, which again, is better offloaded to the gpu then done on cpu.