I have used something similar on an old Fiesta and Focus, both showing P0420 codes even though both cars passed the emissions tests easily (and ECU's up to date).
The Fiesta one came with some metal "swarf" at the bottom to slow down gas flow, which I thought was a good idea too.

In Aidans defence, he is right in what he's saying, unfortunately "OE" spec in the form of emission parts more often than not, is way short of the mark!