First of all:
You can definitely use DSR with a single 980 or 970 GTX. I was using 980 and had no judder aside game engine bugs with DSR and high quality.
You should use 1.1x or 1.2x DSR, that's the limit of a 980 GTX
If you use SLI you can run 1440p fluently and likely even a bit higher. 1440p is 1.33 DSR on the DK2.
I switched from 1x980 to SLI 2x980 and it does help but it maybe introduces a tiny bit of lag, I am not sure so it's ignoreable (and I am very sensible to motion sickness).
For me there is a day and night difference of solo and open play, I am having a ticket checked about that.
So if you experience continuous judder it might not be related to your CPU or GPU, it's likely the open play mode which introduces some sort of latency between the frames.
In my case in some areas it reduces the framerate from solid 75 fps to 50 fps while keeping GPU and CPU cold an didle (20-30% usage).
Hope it helped.
P.S.
If you have GPU performance issues, turn off shadows and ambient occlusion, turn down textures and environment to medium.
On DK2 I could not see much of a difference, if at all. But it removes load from the GPU.