Just for the hell of it, I reset the game to all-round 'Very High' settings with AA and SSAO off, with CSAA overlaid by Nvidia, then I went to fly on ATAG.
FPS warming up on the ground was around 25-35, as soon as I got to 6000ft this went up to 45-50 looking at the ground and soon went to the 60fps v-sync limit.
Looking down later on from 20,000ft there was NO DARK RECTANGLE surrounded by HORRIBLE LIGHT GREEN BORDERS!!!! and frames were steady at 55-60. I also flew around and through a thick cloud formation near France with no huge dip in frames.
The only problem arose when dense Ack-ack surrounded a 109 low down, and when I ran into a formation of 20 Ju88s at speed. Understandable. As you can see from the sig, my pc isn't hot poop by any stretch these days.
But for now, I'm going to leave it on very high, but take the effects down to the recommended 'medium'.
''Job's a good 'un'' as they say (Now we just need the high clouds back).