What resolution are you going to be playing at for the next couple of years? That may be your biggest single concern....I have 2x 980 and the only time i get micro stutter are on badly optimized games which are a very few titles. With that said i am not using a 4k monitor, I am at 2560x1080. Here is a good article at Tom's Hardware that shows recent testing of SLI and resolutions.
 
http://www.tomshardware.c...li-faq,4079.html 

If you shoot high and then go low you will be disappointed in the long run.
If you shoot low and then go midway you will not be disappointed in the long run.

After looking at the specs on Nvidias website for the reference 780ti, you are looking at a 3dmark score comparable to a GTX 970. However, you said you have the classified, so I am guessing that's clocked higher.
 
You only have 3 gigs of VRAM, so that may be an issue if you want to play at higher resolutions such as 1440p or 2160p. But I don't have any data on Witcher 3's benchmarks in this area.
 
1080p should be awesome and not a problem at all, but you may not be able to turn on all the bells and whistles such as ubersampling. They had this feature in Witcher 2 and even today that setting brings GPU's to their knees. It basically is a form of supersampling that renders the picture 4 times to eliminate jagged lines.
 
In my opinion, you do not need to upgrade at all for the Witcher 3 if you are playing at 1080p. You will probably need to upgrade, for 1440p, especially if you are pushing for a 60fps+ game, but the only options are a single 980 or a Titan X. I wouldn't recommend SLI to anyone since it's buggy with new release games. SLI always has the risk of issues when games first come out. If you want to stick with single GPU, you can wait for the 980ti, but there is absolutely no legitimate news feeds on this. Do a google search yourself and you will find the only websites that are carrying any information about the 980ti are rumors.

on a different thought what if i bought a gtx 750ti to use as a physx card since witcher 3 supports physx ? would a 750 ti be enough ?