Your system should definitely be achieving much higher frame rates than 15. I have a very similar setup as yours (2070, 1440p, 32GB, a lesser i7), and consistently getting mid 20 or better at just under maximum rendering, depending on scenery, aircraft, weather, AI traffic etc. 16GB of RAM should be good enough under most scenarios, unless you have very high resolution Ortho scenery and many other apps running concurrent to X-Plane.
One thing to make sure is if you have multiple monitors, have them be driven by the same RTX 2070—i.e., don’t enable any integrated graphic hardware.
Like Greengolfer has suggested, use something such as Windows’ Task Manger to check whether the CPU and GPU are getting their expected utilization.