Right now X-Plane 11 is very CPU bound, however, most of that overhead is due to OpenGL and we are currently working on removing that overhead while modernizing the rendering engine to pave the way for Vulkan and Metal. Right now the best thing you can do is put a big CPU in it, a Ryzen 7 or i7 for example. Once we've got rid of the most offending parts of the OpenGL overhead and also moved to Vulkan/Metal, your GPU will become a whole lot less bored and most likely the bottleneck, especially when drawing lots of objects and doing VR.
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I don't have numbers, only an i7. However, we are trying to move away from wasting CPU, but that doesn't mean CPU usage will be 0. If you think about 90 FPS for VR, we now have a frame budget of 11ms! The more cores you have, the more future proof you will be because neither Intel nor AMD will scale the CPUs much up, but they will definitely get wider and X-Plane will have no choice other than to go in that direction as well.
In terms of long term investments, I would say an i7 is not a bad at all. Neither would be a big GPU, but I'm not sure how much that will help right now.