I was having performance issues running my max monitor res (1680x1050) even on the lowest settings in cfg file, but it's all sorted now, i'm running on the max settings etc, funny thing is it was so simple to fix it eluded me for a few days - changing the res down 1, so from 1680x1050 that's res 1440x900... problem sorted it looks a hell of alot better than at 1680x1050 despite being a lower res.
Maybe it'll work for others. It doesn't just work for ArmA 2, now i can run Crysis at max... woot!. Laters.