After reading some positive reports on another forum I tried the 10.11 drivers and have noticed an improvement. I had been using 10.4 with my 4870x2 since they were released in April and not found any improvements with anything in between but I will be sticking with 10.11 from now on. The aircraft in the Preview screen have jagged edges but I can live with that as in the game and especially in cloudy weather everything is pretty smooth.
I only fly in fullscreen mode so I can't speak for windowed.
Worth a try I think.
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