With some heavy scenery added, I found turning off all shadows improved things considerably regarding framerates...one other item about shadows, ever notice during cloudy/stormy weather settings, all of your shadows still appear as if the sun is still shining? That would be a fix to wish for in the next version of FS.
That and turning off the damn moon reflections on water when there's no moon out...

And cloud shadows on the ground, oh yes that would be nice. Handy for determining wind direction, sometimes (
real pilots don't hit "Shift Z"!)
I've been leaving the terrain shadows on... i have stuttering problems and low fps near cities, but it's got nothing to do with gound shadows.