What? It is? Forgive my ignorance, but isn't it just bitmap rendering? E.g. rending a flat icon should take approx the same GPU toll as a skeumorphic icon
Shadows lead to overdraw I suppose, but I don't believe most UI frameworks draw front to back anyway. They end up accelerating it by making large, multi-element bitmaps of the final render. It ends up being a very small cost.