A little catalytic converter data from Eurosport website.
http://www.eurosportexhausts.co.uk/Catalysts.html
I noticed none of their applications were rated for engines over 350 bhp, but considering we have two primary cats, I assume they'll flow 700 bhp for our N54. All their dealers are in UK though.
from the website.....
Your standard catalytic converter will usually have between 400 – 600 cells per square inch ( cpsi )
Eurosports SPORTS cats have 200 cpsi and our FIA approved RACE cats have 100 cpsi
Both SPORTS and RACE cats will benefit in reduced back pressure and increased flow rates , which is ideal for extracting extra performance from your tuned vehicle
For road use we would recommend the 200 cpsi SPORTS cat to ensure that your vehicle will still pass any emissions laws
The power gains on smaller engines will be minimal with a SPORTS cat but on larger engines you should typically see power gains of between 4 bhp and 10bhp
As a general rule , the more power the vehicle is producing , the greater the benefit will be of fitting a SPORTS cat.....END.