I can't answer your question directly , but the performance of a catalyst degrades over time, especially in a car that is only used occasionally. Therefore a 200 cell cat that will pass the MoT now , may not achieve the same performance win the future.
My +8 OEM cats lasted about 10 years before replacement (and mine is a high mileage car at 3000/5000+ pa by comparison to many) . My second set lasted a similar time. They are becoming rare and very expensive.
IF I could be sure of no roadside emissions checks (and in London they are becoming more common) I would be very tempted to only fit the cats for each MoT and replace them with cat delete pipes for the majority of the time.
