2.5" exhaust vs 2.25 or even 2"

On several cars in the 400hp range, our tests revealed a dyno difference of close to 50hp over a decent shop built 2 1/4" and a 2.5" mandrel bent system.

Wow!
i've seen tests where open headers only added an average of about 12 - 15 hp.
... but that was on a 300 hp 360.