You listen to a Kiwi?
jks, Family is from there, and lived in NZ for a bit.
I'll give you my theory:
You have a hose, Water is flowing from the tap/turbo, exits out the exhuast/end of hose.
You remove finger from the end, flow increases, pressure drops. You remove a bend/kink that is increasing pressue and hampering flow(think cat), resulting in gains. You free up the small fitting(dump pipe) and a heap more water again can flow, and having the larger piping the whole way through allows it out.
On RB's the dump holds them back, compliance cats are tiny(have a look). A cat doesnt flow anywhere near as much as the straight pipes(look up figures), so the more it can flow the better for the whole system. If you didnt notice at this stage from changing, either your cat wasn't overly restrictive(don't know about NZ compliance), or you aren't at a stage where you can notice it.
I personally would do it sooner rather than later, best off to maximise what ever you have, before trying to do more when the weaker links havent been brought up to scratch.
Hope my tired ramblings helped.
-Ryan