Is there a way to clip objects in "letterbox" scaling mode? I can add black rectangles over the areas that I want to appear as clipped, but actual clipping should have a performance benefit. Is it possible?
No, there are no way to do actual manual culling.
On my company we also use black rectangles.
Corona culling, and there are no way to control it from our side, and it cull automatically everything outside the screen. In letterbox mode things in the "black" area of the letterbox are still inside the screen, thus they are not culled.