managed to optimise fps according to several tutorials here, especially the great one from Mystic Puma: (thx tintifaxl for the linking)
http://forum.1cpublishing.eu/showthread.php?t=25543
finally did a performance test with the solo spitfire flight over london, the result was 11/56/101, (textures med, grass on, streets on, shadows off, buildings very low, models high)
didnt overclock my GPU yet, but will probably do for further improvements
flying is really enjoyable now but sometimes the stuttering is still existing...