I think that I read something from Terry Haines earlier this year that the small expansive chamber in the gutted manifold might actually help the exhaust scavenging.
These cats are a lot smaller than the main inline cat further down the exhaust, also it is so close to the exhaust valve exit that the gas could still be expanding rather than cooling and contracting. By the time the gas hits the main cat, it is a lot cooler.
Perhaps that has something to do with it.
I am just speculating as I don't have any experience in this area.
Try to get Mr. Haines to venture his opinion on it again.
warmonger
