[joemosfet]

It is my opinion that the amount of flow between closed and open is negligible. The upstream catalytic converters are much more of a flow restrictor than the perforated and rerouted exhaust.

The picture doesn't really do it much justice, but I think if you were to see this in person, you would see that the path of flow, while convoluted, is still extremely free flowing. The amount of surface area provided by the perforated holes, when all added up, is likely near or even over the 2.5pi (assuming they're 2.5" pipes).

As free flowing as when the cutouts are open? No. Does it make a difference in power? Negligible. Maybe at high rpms the difference would show up on a dyno.

Now, of course all of my speculation is based on experience with several different exhaust systems, as well as knowledge about fluid dynamics, so I could be completely wrong. And, if they were to fill the empty spaces in the muffler with rockwool or fiberglass (often used to increase sound dampening capabilities) my opinion might be a bit different.