Run 3dmark and post the gpu and physics scores.
Make sure nothing is throttling when you run this stuff, too.
edit: maybe dumb, but is windows set to "high performance" mode in the power settings or whatever?
Definitely. There's a 40% difference in GTAV... much more than the clock speed difference can account for. And there are other articles around that compare them at the same clock speed.
Here's an article that specifically compares the 6700K to the 2600K, including SLI at 1440p: http://www.pcper.com/reviews/Graphics-Cards/Skylake-vs-Sandy-Bridge-Discrete-GPU-Showdown/Metro-Last-Light-and-Conclusion
If I were you I would definitely upgrade.
The PC Per guys talked about this briefly in the comments section of their article (see below). It sounds like it's certainly possible but I doubt AMD will get around to it anytime soon. They're 6 months behind when they said the first Freesync monitors would arrive at tech sites, and the CFX...
Frame time analysis is up on PC Per.
The tl;dr is there is definitely frametime variance when you use more than 3.5GB on the 970, but it cannot be said if that is strictly due to the memory issue or perhaps the other differences between the 970 and 980. Also, some games appear to really try to...
Maybe you can buy it from a retailer that will allow returns/exchanges without a restocking fee? I'm not sure if Best Buy does it but maybe look into it.
6950 2GB unlocked/oc'd to 6970, going on 4 years now and it still runs things pretty well (at 1920x1200).
TBH the only reason I plan on upgrading it soon is go to go gsync/freesync.
I currently just have a couple monitors side by side, but if I could have a choice I would go with one large primary monitor and then maybe a portrait one beside it.