RE: Attn: XPS 8700 Owners - Upgrade!!
One thing that you may have to watch out for are temps on the mSATA SSD, mine shipped with none at all, so figured I'd use the slot. Installed the Crucial M550 256GB version & from Day One, the temps of that SSD exceeded all components of my PC, ranging from 50C to as high as 70C (Samsung owners has reported the same).
I felt that to be kind of odd, being that it was directly under my MSI branded GTX 960, which runs at 30-34C, even with a couple of browsers with a dozen or so pages each open, and the MB was normal at 28-32C, the CPU at 32-40C (used to be 28-32C, must be time for re-pasting). At any rate, there was nothing transferring heat to the drive to make it run hotter than the rest in my PC.
A couple of months later, to save the mSATA SSD from what would surely be an early death from heat by first installing it in a SYBA mSATA to SATA-3 converter & I placed it far away from those other components, no difference. Finally, what saved the day & made the mSATA SSD run at temps in line with the rest in the XPS 8700, was a low cost Rosewill 2.5' adapter that came with a small fan, only a SATA to Molex power adapter was needed, once I added that & had the fan blowing direct on the mSATA SSD, temps are finally in line with the rest of the system.
Glad to hear that the i7-4790K is working for you, though it should have ran fine anyway, just some said that the native Turbo Boost to 4.4GHz wasn't working. If the A11 BIOS update fixed this, that's enough reason alone to update it, though I believe the real reason was for Windows 10 support. Still it's good to get to get any added benefit.
My initial plans was to get this CPU months ago, but kept being told 'no it won't work', so having a i7-4770 that hardly breaks a sweat, and as another helpful member on this site advised me, that's a huge upgrade for so little gain. The CPU I have goes up to 3.9Ghz if needed, spending $300 (on promo) for 0.6GHz more doesn't really appeal to me any longer.
The other upgrade I wanted, to have one of the Intel PCIe x4 SSD's, was shot from the sky when I found out what appears to be a x4 slot isn't one at all, plus one needs a Z97 MB to run it, and yet to this day, Dell & other OEM's are still shipping Z87 based PC's. I presume, to clear the warehouse of stockpiled components.
I've came to the conclusion that in reality, there's no more room for speeding up the XPS 8700 based on the fact that I have a Z87 MB, and that SATA-3 SSD speeds will be the bottleneck for any upgrade that I perform. No add-on card is going to change that, it's further hampered by having only one graphics slot, which prevents me from pairing a couple of future GTX 960 Ti variants that will likely have a 192 bit bus option at a minimum.
While the XPS 8700 has been the best PC that I've owned, with an i7, 12GB RAM, 1GB GDDR5 GPU (the latter two has been upgraded, in addition to 3 SSD's) for all intents & purposes, it's maxed out. Moving to another case won't do anything other than allow me to upgrade the PSU to add a single GTX 970/980 (& BTW, watch that PSU, I've see YouTube vids of a 970 install, but not a 980). I'm not spending $500 (minimum) to upgrade a GPU when the one I have meets my needs, have DX12 with Windows 10 Pro.
My next PC will be self built & not with a 4 core CPU, rather one of the 6 core Intel offerings, I don't need built in graphics, for less than $50 more than the price of the i7-4790K, can have one of Intel's 6 core offerings, though I'd want to go a bit higher.
Hope that you enjoy the i7-4790K & GTX 980!
System Specs, as provided by Speccy (05/22/2018) Delidded i7-4790K!
XPS 8700 specs (current), added 92mm front intake in well concealed stock area, no case mod needed, only two screws & PWM splitter (exhaust tied to CPU fan speed), intake uses stock exhaust header & fan. All fans controlled by SpeedFan, otherwise would be locked at just over 800 rpm, now all runs over 2000 rpm.