Found this interesting info graphic from a Corsa performance ad on a Lingenfelter site:
The graphic shown is for the 14157 system versus the stock 2005 Cadillac STS system. This is particularly interesting for the STS-V since this is the same Corsa system that D3 sells for the STS-V.
The text notes: * Flow bench-tested 42.3 gain in exhaust flow vs. stock muffler.
This I read to mean that the graph is for flow through a single Corsa muffler versus a single stock muffler. So in other words, the dual Corsa mufflers probably flow around 437 x 2 =874 CFM and the stock mufflers flow around 308 x 2 = 616 CFM. Recall our target is 2.2 CFM/HP for 0 backpressure, so with a modified STS-V making 500 hp plus S/C load we would need 560 hp of exhaust or 560 x 2.2 = 1232 CFM.
I am guessing that the STS-V stock exhaust flow is similar to the STS stock exhaust flow.
We can see from the numbers then that the stock exhaust flow is insufficient, and that the Corsa’s are an improvement, but there is still some margin for further improvement.