Yeah the stock cat is incredibly restrictive at 600cell, also the diameter of the downpipe is around 50mm which is much smaller than the rest of the exhaust. Lots of power to be gained by fixing these two issues.
I bought a decat full supersprint exhaust with the centre muffler deleted. I put a HJS 100cell cat where the centre muffler would have been. It sounds amazing and there was a noticeable increase to the power. That being said its much easier to get away with running a 100cell cat in Australia.