Colin,
Give me a bell and i will give you the full lowdown on this, but short answer is yes it would pass an mot.
The cats i am looking at are more efficient than the std iten as there design is newer, the matrix is 200 cpsi (cells per square inch) and std is around 600- 800. the full race cat is only 100 cpsi and would not pass an mot.
9bear in mind that the original cat design is over 15 years old, we have moved on)
Yes, these would be noisier, but not a great deal. At a guess , more of a crackle or rasp at higher rpm when the fueling goes open loop.
There is a lot of argument in the real world about this with the latest mot update, i would not be prepared to get anything made that is inferior or does not work, i would rather walk away from that, this is why my initial price is so high.