Have you considered using World Size only for the objects that need it, by using the Corona Displacement Mod? That way you can have screen size for most things, but the hero objects or ones that don't work well with screen size can have the benefit of World Size.
Also, are you using 2.5D displacement, and using AutoBump, both of which should let you keep displacement "lighter" without losing detail (so you could either a) switch to World Size because it has less impact or b) increase the Screen Size accuracy in the hope that it removes the flicker).
The thing about Screen Size is the savings come from exactly this - displacement is calculated on-the-fly based on the camera, which lets the scene be lighter by calculating a lower detail of displacement outside of the camera field of view. The downside is that since it's on-the-fly and recalculated each time, it can change as the camera moves; so it's an inevitable part of Screen Size, the recalculation is what lets it be lighter, but also what means it can change between frames. So the benefit and drawback are tied together :)
