I removed the stock header on the NC the other day and took some pics of the "squid".

On the internetz this piece is considered to be the most restrictive and badly designed part on the whole car. I partly agree but I also understand some of the challenges Mazda engineers had when it was designed;
- Cramped space
- Pre CAT needed to meet emission requirement in all markets including California
- noise
- mass production AND low cost.

At first glance the pinched runners looks to be the main restriction:

But from above you can see that the runner are pressformed into a square on the inside which means the volume of the runner is not that compromised as it looks.

The obvious restriction in the header is the CAT but also the cramped "collector" that houses the O2 sensor:
It narrows down to about 2"/50mm and are shaped around the O2 bung. I wonder if this is to optimize the O2 sensor reading or optimize flow into the CAT.  For a comparison look how Porche did it on the 996 turbo exhaust. Turbo is dumping exhaust from the turbine directly into the CAT via 4" opening:

The CAT outlet seems very good and has a smooth transition into a 2.5 exit. It is also smooth on the inside with no edges.

Runner flange looks good with a taper from the gasket ID to 38 mm ID runners.

Any serious performance tuning on the NC will involve replacing the stock header.  I will recommend swapping it for the  Racing Beat header or the NOPRO header if money is no object.