They certainly do work but they might not work for you. One reason is that you might not be able to get a set of headers built for your car that both fit and work. The primary pipes need to be really short to use the 421 collector and that can be hard to package into a car.

Another point is that the guys who pick up power with the merge headers are typically doing something different with the head and cam. One example is using the header design to pull harder on the exhaust. Once you do that you can put a smaller exhaust valve in the head which then lets you put a larger intake valve. This is the trick that NASCAR guys did a few years back and it is the trick that some pro drag race teams use. But it is super expensive for the average guy to try and do these tricks.