If there is doubt on the OGL FPS, I would go to the X-Plane’s dataref to for the “official” number (under Data Output Settings).
After upgrading the former bottleneck (750Ti), one would justifiably expect the performance to go up, at least by some noticeable amount. And as you’ve said, with all the Graphics Setting at minimum, the FPS should be better than 25.
Hence I can only speculate that there exists some interaction between the CPU/RAM and the GPU that is causing the FPS problem. Perhaps the GTX 1060 interface is now demanding more processing power, than what the 2.7GHz i5 can “catch up” with. I have seen a case where an X-Plane simmer had upgraded his GPU, but yet the FPS remained unexplainably low—until he had upgraded the CPU as well.
This may require someone with actual X-Plane programming expertise to explain.