Witcher 3 is very demanding of the video card. In other words, yes, it makes the video card work hard.
To be honest with you, I doubt that the CPU is holding you back. I suspect Witcher 3 is just rather poorly optimized.
As for upgrading the video card, the 770 is not a bad card. I still have the same card in my system. There are going to be new 14/16nm GPU's out in less than a year. I expect them to be much, much faster, and to have a lot more memory on them than the cards that are on the market right now. I am going to wait for those to come out, and for the market (prices) to settle down some before I upgrade. I cannot afford a new video card every year, so I try to upgrade when doing so is going to get me a very nice boost in performance.
Now, having said that about the video cards, I am hearing that those new video cards are going to have previously unseen amounts of memory bandwidth. And I do mean massive amounts. Currently, the highest memory bandwidth is 512Mb/Sec. Some of the new cards will have 1.2Tb/sec. Which is going to really make them fast. And that will probably task your CPU harder. So while I do not think it will make a whole lot of change for you today, it will when you get one of those video cards next year.