[klem]

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EDIT:
My original settings (first patch) were High with Building detail set to Medium, Forest set to low, Buildings Amount set to Medium, Grass Off, Shadows Off.

This gave me 25-35fps low over London with occasional short dips below 20 and minor stutters as major features loaded.

Generally this patch, with SSAO checked and Textures at original (= previous settings?) has improved frame rates by removing the lower end low over London as it dosen't fall below about 35 although the upper 35 has only increased to about 38-40. A general improvement of about 5-10 fps with the bottom end raised.

So on these settings the patch gives me (all at Rooftop level over London):
Suburb Rooftops 35-38 (was 25-35)
Approaching Tower Bridge over the Thames ~37
Through Tower Bridge ~34
Approaching Houses of Parliament ~38
Over rooftops of Houses of Parliament~38 with a 14fps spike right alongside Big Ben
Over Buckingham Palace ~29

As new distant textures loaded for Tower Bridge, Houses of Parliament, Buckingham Palace there were spike drops in fps, too quick to measure but probably down to around 20.

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Tried Very High settings.
Approx 20-25 over London, occasional drops to 10-11 as some majore features loaded like Houses of Parliament.

Then tried:
High settings, except Forest low, Buildings Amount Medium, Grass etc on, SSAO Unchecked, Textures High, AAx4
AFx8
AA Gamma Correction OFF
AA Transparency OFF
CUDA GPUs - ALL
Max Pre-rendered frames - 3
Single Display
Power Managment - Max Performance
Texture Filtering LOD Bias CLAMP.
Texture Filtering Quality - Quality
Texture Filtering - Trilinar Optimisation - ON
Threaded Optimistaion - AUTO
Triple Buffer ON
Texture Filtering Aniso Filter optimisation - OFF
Vert Synch ON

ASUS Sabertooth mobo
i7 950 @ 4GHz
6Gb DDR3 RAM
EVGA GTX570 GPU
850W PSU
Coolermaster HAF912+ case
Samsung 22" 226BW @ 1680 x 1050
X52 HOTAS
X45 miscellaneous controls

EDIT: Samsung 226BW 22" 1680x1050

FPS, London low level (rooftops):
Suburbs ~35
Approaching Tower Bridge ~24
Through Tower Bridge ~35
Approaching Houses of Parliament 25-35
Over Roofs of Houses of Parliament 30 with a spike of 17 exactly at Big Ben
EDIT: Slightly lower than those Edited in above for my old settings even though Textures were Medium and SSAO unchecked but the Grass and Shadows were Off.

Quick Missions:-
Country Terrain 6,500 feet 65-75
Country Terrain 200 feet 40-50
Water 6,000 feet ~80
Water 200 feet ~60

During the first test run the sound went intermittent after about 6-8 minutes. It restored when I re-loaded the same mission. Didn't happen again.
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EDIT:
Final Settings:
Very High, Building Detail Medium, Forest Low, Buildong Amount Medium, Grass Off, all low level ~ rooftop height
Low over normal Terrain ~37
Low over medium-high forest concentration 31-37
Suburb rooftops 26-28 with a spike to 22
Approaching Tower Bridge ~24
through Tower Bridge 24-30 with a spike at 19
Approaching roofs of Houses of Parliament 27-30 with a spike to 17 over roofs (in Westerly direction) and a spike to 15 exactly alongside Big Ben (in Easterly direction)

General fps is good, smooth over london except the spikes when some textures load.