Hello Piecemeal,
I also have slow downs over Hanoi and when there is a lot of action on the ground and in the air.
I changed my graphics card from a Radeon 7770 to a Radeon 7870, but dit not notice any difference.
I gained a big boost from disabling mirrors, and putting ground objects on medium.(I know you dont want to change these settings)
I also disabled HPET in the bios and Turned DEP off in Windows.This gave me a huge boost in DCS A-10 and a small one in SF2.
So the bottle neck is my CPU.
These are my settings.(my system is in Dutch)
CPU intel i5 2400 3.1 GHZ
2x OCZ 120 GB ssd
Radeon 7870
8 GB Corsair RAM
Catalyst settings
INI settings
Game settings:
This is what is lookes like in game: (still a lot of buildings and trees)
I hope this information is useful to you.