Just wait till you get the Euro Cat VI with NOx sensors and treatment injectors in exhausts, just getting an exhaust fitted will be trouble. As for EGR’s they are a botch just to try and reduce NOx’s being generated by loweing the temperature of the combustion chamber.
Checking for Cat’s during an MOT is an interesting item, if the shape of the exhaust section is unchanged and the emissions are fine, it’s very difficult to find a reason to fail it. Subaru’s with straight pipe the size of a flue for an aga and emissions well off the scale are easier to spot.
Just because it is proposed for inclusion to the MOT test doesn’t mean to say it will make it into test criteria, there was a big thing a while back about putting window tinting in, but the industry kicked up about buying extra equipment and calibration that it was chucked out and remained an item for the police.
As for catalytic converters failing don’t see many, but in the main coil packs that have gone down, so lots of unburnt fuel making it into the exhaust and poisening it, or cheap replacements. Some crack and break up as well, old mondeo’s seem keen on this.