Really hope this isnt a daft question but what benefits would you get from having an oversized cat?
Genuinly do not know this
It maintains flow and keeps it on level terms with a decat.
With a 'normal' size sport cat (I won't name other exhaust manufacturers, but you know them) you may lose flow in comparison to a decat pipe, simply due to design restrictions.
By increasing the size you get around this problem.
Plenty of us don't want the potential hassle of a decat pipe at MOT time, so it's really good that the Powervalve cats are a worthy option in terms of performance potential.
If you get the chance to look at any dyno graphs of cars with Powervalve sports cats you'll see just how well they flow. The curves are incredible, especially with a bit of R-Tech magic.