Had been considering replacing my SLI 670 FTW's with a 980 SC (rear exhaust) on my X58 system but couldn't find any performance posts matching this exact hardware to see if it was worth the plus $500 .... so took a leap of faith and bought the Evga 980 SC direct from Evga. 
 
Here's my results for anyone with an X58 mobo and similar hardware in my signature and wondering if it's worth it:
 
3DMark Firestrike 1.1 score with SLI'd 670 FTW GPU's (with my subsystem hardware and 3.6 GHz CPU overclock) = 9926
3DMark Firestrike 1.1 score with single 980 SC (with my subsystem hardware and 3.6 GHz CPU overclock) = 10,975
(the separate category graphics, physics and combined scores with the 980 SC were all around 1000 points higher, same as the total 3Dmark score).
 
I briefly bumped up my CPU overclock to 3.8 GHz to see if there was an appreciable benefit and the answer was "not so much", i.e., a modest avg. 100 score increase across the board with a 200 MHz CPU speed increase.
 
I had hoped for higher benchmarks than what I got coming from dual 3 generation old 670's. So, for plus $500 it boils down to a ballpark 1000 point 3Dmark benchmark increase but with a single GPU w/4 GB of VRAM that's DX12 compatible down the road when W10 games appear with it (but will stick with W7 per my sig until there's a benefit to fix something that ain't broke other than DX12 capability).
 
Again, this is just a FYI for others with similar hardware to help decide whether to spend the $$ to upgrade from SLI 670's to a single 980 SC GPU.

A single MSI GTX-980 Gaming 4G at stock clocks beat both my Firestrike and 3DMark11 scores using two overclocked EVGA GTX-680 in dual-SLI but in the individual results my 680 SLI had better FPS by as much as 5 to 8 frames per second in every category.
 
It took me a while but here is my comparison: http://www.3dmark.com/compare/fs/3146666/fs/3213035