Its pretty terribly optimized. 
 
But. There are a couple of things you can do.
 
For SLI, force AFR 2 in Nvidia Control Panel. I think this should let you use SLI to some degree. Download the WORSE Mod for the game. It will increase the framerate a little bit, and it will reenable all the E3 settings that the main game took out to make the console versions look better.

I am a little bit confused. You stated you have a 4790k, SLI 770 4gb, and 12gb of Ram..

The 12gb of Ram is such an odd number, lol.

You stated that you were using the optimized settings.. What are you using to optimize them?

God no! Get rid of GeForce Experience! That thing is terrible and implants bad ideas into games.

I would suggest completely uninstall in your driver's through the control panel and restarting each time it requests you to. Once you get everything removed, reinstall using the custom feature, then select Only the PhysX option (you can't unchecked the driver, so... You are obviously installing that, lol) remove the audio, GeForce experience and 3d stuff.. Click Clean Install on the bottom left of the dialog box.

Once everything is fresh installed, you may have to delete the registry files the GeForce experience puts in the game.

If you have got to have shadow play, install geforce experience, from scratch, and unchecked all of the auto update, game optimization and other garbage options it tries to overburden you with. Simply use the feature you Have to Have.

Once that is done, and everything is clean, the try setting up the game yourself. Don't let software push a file in the helps destroy games ;-)

I am NOT saying that it will make the game perfect, but GeForce experience is terrible.

Update!
 
After doing some digging, I've found that the real problem with the software besides optimization is the SLI profile.
I will be trying some things in the NVIDIA Inspector over the next few days, but I've gone ahead with the following:

• Create a new profile in the NVIDIA control panel
• Turned SLI off to single GPU usage only
• Quality settings at High (not Ultra)
• Texture setting at Medium.

I'm now able to play the game consistently at 30FPS (minumum, 60FPS when not raining or action). I'll post any SLI change results I find over the next few days but not using SLI could be a way for many people to actually play the game - assuming they want to haha...
 
I've heard of SLI not being the best option opposed to single GPU for a variety of reasons, but games which perform worse with SLI on is a new scenario for me...

kidcrumb
I dont think thats true. What hardware do you have again? I have this game maxed out and I get 55-80fps.

I'm running 2x EVGA GTX770 4GB SC.
If I'm missing something, I would love to hear it...

fubarhouse

kidcrumb
I dont think thats true. What hardware do you have again? I have this game maxed out and I get 55-80fps.

I'm running 2x EVGA GTX770 4GB SC.
If I'm missing something, I would love to hear it...

 
For SLI: 
 
Update your drivers to 344.75 because it includes an SLI profile for the game. Make sure that you have SLI enabled. After each new driver installation, Windows likes to disable my SLI and I always play games and then notice its not turned on.
 
Update the game to version 1.3
 
If that doesnt work, use the SLI bits for Far Cry 3. This will get SLI to work AND you shouldnt get the black shadows problem, but it comes at a performance cost. You wont get as good of SLI scaling as you would with the Far Cry 4 SLI Bits.
 
It also helps to OC like a madman. My cards are OCd to 950Mhz. Stock clocks for the GTX580 is 772mhz. Thats a fairly substantial boost.