Now that I am able to run 4K easily, I decided to try NVIDIA DSR again. My current monitor is a Yamakasi Catleap with overclockable PCBs, until the PG27AQ releases.
 
But in my case, there is an issue with using a refresh rate over 60 Hz simultaneously with DSR. NVIDIA does not allow custom resolutions to be used with it - but you can! I toyed around with this for awhile before figuring out how to make it work in SLI... hopefully the same is true for you. Skip past the screenshots of the game for info.
 
I played a few games and took screenshots with DSR on and off, and 4K makes a noticeable difference in details, especially when rendered at a distance. But even moderately close objects benefit. In BF4, I noticed that SSAA (or supersampling the rendered resolution of 1440p) produced a sharper image than did 4K DSR. Rendering in 4K had more detail when cropped as an image, but SSAA 1440p looked better and was more of a performance hit depending on the level (200% used 4.5 GB of VRAM at 60~ FPS, 4K used 3.5 GB at 100 capped FPS). This is likely due to the blur filter used, but I compared at 15% and 0% smoothness in the control panel for DSR, and it still looked worse. For games with SSAA, I would recommend that over DSR.
 
Here are some examples from a screenshot in Watch_Dogs. I cropped these, not perfectly, but full screen at 1440p. The 4K original images are much sharper than what I am showing as comparisons, but remember, we want to see what 4K looks like downscaled onto 1440p, which is what I have done here:
 
1440: https://farm9.staticflick...057_beb9085dcb_o_d.jpg
4K (below): https://farm8.staticflick...944_2411c20733_o_d.jpg

 
1440

4K DSR

Notice the bars in the fence being straighter and not jumping around as much. The red sign also has missing elements in the 1440 shot. The blue mortgage sign is more legible in the 4K shot.
 
1440

4K DSR
 

The chain link fence is obvious with differences here. Apart from that, the car is smoother and vegetation less blocky. The bars on the fence in the bottom right do not break apart as they do in the 1440 render.
 
1440

4K DSR
 

Text on this sign, at fairly close range, is sharper and darker.
 
Again, these were all crops at 1440p resolution. These are differences you would see on-screen in-game.
 
So, how did I do it? You need:
-ToastyX FULL pixel patcher (for SLI - shouldn't be necessary for single cards, but you may need the standard patcher if your monitor goes out of zone above a certain Hz)
-CRU 1.2
 
1. Have your GPU drivers installed. For SLI at this resolution, use the full pixel patcher and reboot.
2. Start CRU. Select your display you will be using (selected at top), set the extension block at the bottom to "Custom extension block" - do not add resolutions/refresh rates here, otherwise NVIDIA will recognize them just like their custom refresh/resolution utility.
3. Add custom resolutions/refresh rates in the detailed resolutions block with 4 total available slots (3 if you have 59.95 Hz enabled)
3.a. ***You may need to test what you can hit before your screen goes black/drivers crash. For me 99 Hz is the maximum, even though my monitor would run 119 Hz without DSR.
3.b. ***I deleted 59.95 Hz/60 Hz because of certain games, like CS or Evolve, where you cannot select refresh rates in the options. For me, it would default to the lowest refresh rate when games booted up. In something like Battlefield 4, you can pick from all your refresh rates so this isn't necessary. Borderless window is not really a workaround because performance will suffer, especially at 4K where you need full GPU utilization.
4. Click copy in the top right, and OK.
5. Use the restart64.exe in the CRU folder to restart your GPU driver.
 
This is what I have mine set to:

 
 
-With this setting, my monitor only displays at 99 Hz, at any resolution. On the desktop, it will display 1440p at 99 Hz and downclock my GPUs. If yours don't, you may need to increase the backporch timing manually.
-When switching to DSR 4K, 99 Hz is still the only option so it forces that refresh rate. This works in all games at full screen.
-I started at 61 Hz and gradually increased until my driver crashed/screen went black. This occurred for me at 100+ Hz. I first tried 99 Hz, and it crashed, but it worked once I slowly creeped up the numbers from 61.
 
Here is where I need help...
 
I can't figure out why my monitor blacks out and the GPU driver crashes at 4K DSR over 99 Hz. I also can't figure out why I need the pixel patcher in the first place, particularly in SLI. The GPU is still outputting 1440p at X refresh rate (pixel clock itself - right?), but is rendering at 4K. Maybe this is why it needs a patch for SLI as somewhere in the driver this is limited, or thinking it's outputting in 4K when it isn't - I'm not really sure.
 
Nothing wrong with 99 Hz... pretty darn good. But still, without G-Sync, higher refresh rates help reduce tearing, so I'd like to go as high as possible. And yes, two TITAN X's can run most games at 4K over 100 FPS.

DSR is supposed to work out of the box with SLI. Make sure you do not have any custom resolutions in use, and it should be in your 3D settings page. I know mine was missing for that reason initially, but I think it was also missing randomly just because I had SLI enabled. You might try reinstalling the drivers or Googling around... I know others have had the same issue.