Not sure which hi-flow cats you have flow bench tested, but when looking at the brick in my N-Tech hi-flow cat and comparing it to the stock cat's brick, there is a substantial difference in the honeycomb "density" (number of holes per square inch). The stock cat's brick may be larger and have more core surface area, but the hi-flow cat (at least the N-Tech I have) has better density and also less core length, which affect flow.

Every two years when I have to get my FD smogged, I reintall the stock cat, but leave everything else in the exhaust intact (DP and Racing Beat CB). There is a noticeable difference in power, both in response at lower rpms, and at higher rpms/boost after the secondary transition, and boost ultimately reaches 1 psi or so higher.

 

When I went from the stock cat with almost 100K miles on it to a Bonez 3" high flow I noticed a large difference in power and the power delivery was much more smooth and responsive. The stock cat also weighs a lot to say the least so the weight savings is another great benefit to the high flow unit. I have passed Cali smog twice with the 3" cat with below average emissions each time.

 

I had a slight out-of-spec smog test with our original cat in 2002. None of the "standard" fixes (plugs, wires, making sure engine was hot, etc, etc.) worked. Replacing the old OEM cat with a brand new Bonez high-flow resulted in a way out-of-spec result. Buying and installing a new OEM cat fixed everything. I think the air injection of the OEM is carefully designed inside the cat to make sure all of the active catalyst "sees" the injected air. The Bonez probably just blew air into the casing without distributing it properly.

 

I passed just fine with a new streetported motor (200 miles) and the Bonez hi-flow cat....

 

Both times I passed smog with the Bonez cat I didnt run a pre cat btw...

 

all this talk, i just pay a guy to pass my car during inspection time.