Brzezinski cuts them open and closes them again, yes it's not legal. Some of Vizard's work has pics of Wilson-modified SBC dual plane to both improve flow and equalize runner CFMs, not what you'd expect. The upper vs. lower plenum characteristics (height and volume) make this tough to do.
I generally radius anything I can reach, do some directional blending where it looks like the flow "wants" to go, but nothing major - I just don't know enough.