Things to try if you haven't already.....
delete/rename the files profileoptions and profileoptions_options located inside your DAI saves folder....this resets all in game options to default and sometimes fixes strange things.
Go into Nvidia game settings and try the "optimized" setting for DAI and if it is already there, try custom with settings fairly high.
Resize your display in computer settings, IIRC it showed you running 1920x1080 60hz and natural setting for monitor was IIRC twice that, so resize it up to the 3940x2160 60hz and retry after reboot....resize back if you don't like it and recheck after reboot.
The issue as I see it is in the GPU not ramping up, which I would say is usually driver issue, or maybe interference by another program....such as fraps, teamspeak, geforce overlay, etc...
Even with SLI good cards you would expect to see 50-90% GPU use unless capped frames lowered it for you, as most times my FPS is capped at 60, and just in DAI my MSI 290X lightning was pulling between 60-90% useage. While it isn't near as good as your twin cards, it still shows that running fully your cards should be able to do it, BUT they aren't being pushed up to do it.