talking about exhausts,

here in NZ, the government is passing a new law...
that says...

All exhausts must be as quiet as factory

i just got myself a Kakimoto exhaust system...
from reading this post, i think it should be a cat back system.
i was wondering if i keep my Jdm Cat, would my exhaust sound be a lot more quiet than having no CAT (in NZ, we do not require Cats)???

and what is the inside diameter of a Jdm cat?