I personally do not think so. While X-Plane is more dependent on graphics card power than FSX, for example, it will still quickly reach the point where it is "CPU limited". I have upgraded from a GTX770 to a GTX970 and have seen no increase in FPS at all. It is worthwhile to have more Videocard RAM, so you can run at a higher resolution, though.
The number one way to get more performance, once you have a fairly decent videocard, is to add more CPU horsepower.