Are you measuring those FPS inside MSFS (ie. dev mode FPS view) or externally with another utility? If inside MSFS, be aware that the dev mode FPS does not show your doubled FPS correctly with frame generation enabled as it is the GPU driver rather than the game that is doing the doubling.
FWIW, my 5800X3D with a 4080 can show as low as 35 FPS in game at 4K Ultra in a complex scenario, but with frame generation that doubles to 70 FPS when checked externally, which is 10 FPS more than my 4K60 TV can display so it is all good from my perspective.