I have my texture resolution, HDR, Anti-Aliasing, water reflections set pretty high, with a little bit of weather. This primarily chokes my GPU, pushing it to around full usage all the time. Thus, I have already determined my GPU to be the "bottleneck" if there were to be any, because my CPU is not running near 100%, which technically means I still have room for drawing more objects, cars, and a higher Field of View without (in theory) losing FPS.
And yes, my GPU is DEFINITELY less than your 1070 8GB VRAM. Let these links do the talking:
Plain and simple, the GRX 1070 is one of the best on the market for its price. Scratch that. It is THE best card on the market for the price. I would be excited about it.
As for your actual concerns... the CPU should pull along just fine. Now, it is just a quad core with no multi-threading at a maximum of around 3.6 GHz, which should be fine SOLELY depending on the settings you have for "Rendering Options". Thus, if you experience problems with it, you would have to compensate accordingly... and here's a great link for that:
Now, I don't expect that GTX 1070 to have problems running X-Plane. If it does... than you can blame X-Plane lol.
As for my situation, I have been hoping to overclock my CPU to around 4.4 GHz or 4.5 GHz, and maybe in another year or so I want something from the GTX 10XX series.
(lol meanwhile I know people with like 8 multi-threaded cores on an AMD chip).