Oh, a follow up to the whole the "the stock flows fine" discussion, and that it's already high flowing, mandrel bent, etc.
It has a flapper valve in it. I was getting my old OEM ready today to try to sell it (anyone want to buy an OEM exhaust in great condition?) when I noticed a little knob looking thing. I played with it to see what it is/does.
It's literally a blocker valve thing on a spring. Like a throttle body plate. Presumably when there's enough exhaust flow, it forces the valve open. It's positioned right after the mid pipes, before the central muffler/resonator thing.
The valve closed, with a flashlight on it (it has a little hole in it):
The valve open half-way, I'm twisting it against the spring tension:
The valve just about all the way open:
There's a little ball on the opposite of this plate, it looks like a flapper valve from a toilet. This device is called an Adaptive Valve, and its job is to attenuate low frequency sound in the exhaust. It works to keep the exhaust quiet. These valves do increase backpressure, though they maker claims only a "moderate" increase.
It's worth mentioning that aftermarket exhausts, such as ones from Thermal R&D, do not have these devices in them.