No, flat shadows gives best performance, I just find it too noticeable ie. no active shadows in cockpit and no shadow on carrier island, so I go with low. Diff between low and high is less noticeable for me.
EDIT: Forgot there are two different settings, "shadows" and also "terrain object shadows". I believe "TOS" is the one if set to flat makes the carrier island not cast a shadow. If "shadows" setting is set to flat, that is the one that removes shadows from the cockpit.
I had "shadows" set to Low and "TOS" set to default, going to see if I get an FPS bump setting TOS to flat. There is a really good comparison video of all graphics setting, (link below) and I don't see much difference between TOS setting of flat or default but about a 10 FPS difference.
DCS 2.7 | Side-by-Side Comparison