Now that I got it running (thread in Technical threads), I'm seeing a small increase in fps overall, but still some major stutters over London while it seems to be loading in stuff. Post stutter, it's a flyable 20fps at 1680x1050, with model, damage, and detail high, rest low/very low. No roads or grass.
Grass seems to impact quite highly in the countryside (as you'd expect there being more of it
Quick flight, even dogfight over sea, averaging 30 fps 6v6 with clouds (which in that one are not only beautiful but seem to eat frames).
I get a stutter at the start of missions when airborne, when again it seems like its loading stuff in, but thereafter it's ok. The exception being, once again, London. Quick mission, bomber intercept, is about 12fps with no clouds or trees.
Application crashes just flying around have increased a lot with unhandled exceptions.