The purpose of an EGR is to allow some exhaust gas into the combustion chambers under cruise and decel in order to displace some oxygen and fuel. Aka .. helps with mileage. You likely won't notice a huge drop in mileage though.
As for codes, that's easy enough to work with. I have both DHP and HPT and can easily do that. Hell..if I didn't want my egr to work and knew I needed it for emissions visual inspection (depends on where you go in mass) I'd simply tell it not to run. You could also block the tubes, but that's more of a pain then if you say had the LIM off and decided to plug it internally. I did on the Bonne with JB weld due to an IC. Right.. IC with EGR sitting there..classic.
On the cat, you need a cat that will work for you. I'm not as up to speed on which ones will work for the cat diagnostic test. However the codes can be tuned to no report. The diagnostic still runs and if the cat isn't up to par.. you'll run super rich due to the diagnostic continually running.
