This is what is working for me at the moment, I am still experimenting. I set up a mission with the Huey at Carpiquet and fly over Caen at the same speed and altatude, changing settings each time. At the moment I am getting 32 FPS over Caen, and about 60 FPS over the country side, and just a little bit of shimmering if I look for it, I can eliminate the shimmering, but frame rate dropps to about 17, and that's annoying. I tried Textures and Terrain on verious settings and they made no difference to FPS. What did make one of the biggest improvements was my monitor resolution. I changed it from the native of 2560x1440 to 1920x1080 and I got an extra 10 FPS, when I start a flight, I press Right Alt and Enter to go into full screen mode. So that is where I am up to at the moment. I am starting to play with Preload and Trees now to see what impact they have.
Hope this gives you somewhere to start. I like to set the settings low for good frame rate, then increase each one to find out how high I can go before the frame rates slow down too much.