This hardly matters. There is a 0-1% drop moving from PCIe 3.0 x16 to PCIe 3.0 x8 on a 980.
Even moving to PCIe 2.0 x8 / 3.0 x4 is just a 4% drop at 1440P.
Upgrading from 970 SLI to 980TI SLI would net a massive performance increase.
As an example in TW3 a single 980Ti ~ 980 SLI
Or when SLI doesn't work at all, 970 SLI, the performance delta between 980Ti and 970 is massive.
Essentially a single 980Ti OC ~ 970 SLI OC.
As far as your example of using a PCIe SSD in the rig, a 512GB one costs almost
$400 US. Chances are someone who can afford a $400 512GB PCIe SSD, $200 board, $350 CPU can afford Skylake-E. Since IPC increases are slowing down for Intel, while DX12 will bring more emphasis on multi-core scaling in games, a Skylake-E 5820K successor system should give a gamer a peace better performance over the next 5 years. Even with 5820K,
more members of AT voted for it over i7-6700K, nevermind Skylake-E. Some have tried to argue how overclocking X99 CPUs isn't guaranteed but given the $350 US price of 6700K, it's not a big stretch to get a
$419 binned 4.6Ghz 5820K.
I think patience will reward Skylake-E buyers since those CPUs should overclock similarly but run cooler since they will have proper solder. Over the next 4-5 years, having 2 extra cores for barely more $ is going to pay off as well. I mean at that point in 2019-2020 we could even get PS5/XB2 consoles. Since it's already August 2015, it's not out of the question that a Skylake-E will survive to next gen consoles. Who is going to be that there will be less than 3 AAA games that benefit from 6-core CPUs over the next 5 years? We already have some games today that benefit. I remember how the entire forum kept recommending i5 over i7 when I got my i5 2500K and I hate to say it but most of those people were wrong. Learning from past mistakes, I think it's going to be history repeating itself.
I am also frankly amazed how many PC gamers on AT are hyping up Skylake upgrade over 4.5Ghz SB/IVB when almost none of these individuals even have a single 980Ti, nevermind 980Ti SLI (or Fury X CF, etc.). Sounds like a lot of people are just looking for excuses to upgrade, which is fine since upgrading is fun but it's doesn't change the fact that it's still not logical when it comes to
gaming performance upgrade priorities unless one's monitor and GPUs have been upgraded first. CPU upgrades no longer mean what they did 5 or especially 7-10 years ago. Today, it's all about the GPU(s) and monitor ugprades imho when it comes to gaming experience.
Not to mention, most of the gaming benchmarks online comparing Skylake to previous generation of CPUs aren't using highest in-game settings, skimping on AA as well, or using low resolutions. That's not how most real world gamers play their games. Most gamers do not buy $600+ of GPUs to play games at 1080P non-maxed out. In fact, most gamers will use DSR/super-sampling methods when there is enough GPU horsepower which will make the system even more GPU limited.
When comparing real world gaming situations of proper AA/highest settings, the performance upgrade from even SB i7 2600K OC is trivial.
http://www.bit-tech.net/hardware/2015/08/05/intel-skylake-review/7
Raise the gaming resolution to 1440P and upgrade to a quad-core Skylake is going to be even less.