Dilution rate would be the only thing I can think of that would move the maximum values up or down proportionately with the corresponding change in dilution registered by the sniffer at the tailpipe. I'm not up on my flow dynamics but maybe just the simple installation of a slight bottleneck can help in a small way to register better numbers with an otherwise stock or modified system that is left untouched, save for the catalytic. That's good info about the catalytics all the same Steve, I just hope the cars were not changed other than the catalytics to make the comparison more than fair.

POINT OF NOTE FOR NEW CATALYTIC CONVERTERS - They have a break-in period where they actually season and are supposed to convert more emissions in/out to lower numbers. This number can very from 100-500 KM's of driving but it is usually stated in the paperwork you receive with the new product. It may help you, and other than burning gas you would otherwise use in your daily travels, it's a little more piece of mind.