I've read most of what Calvin has posted on 4-2-1 headers and merger collectors and I think it boils down to building an effective extractor type header system which then allows the engine builder to use a smaller exhaust valve which in turn allows a larger intake valve. So not a silver bullet at all! But the NASCAR guys have used it to pick up some power and I'm pretty sure those engines are tuned now to a point where a standard off the shelf type of header would kill off a bunch of power.

The big trick seems to be cutting the primary length way down to improve high speed performance. Then the merge and choke are used to make it work on the bottom end. This is why just welding a merge collector onto a standard set of pipes doesn't do much. It doesn't start to work until the pipe length is correct which takes some math to figure out.