Standard cat vs Sports cat with Janspeed manifold

Before people get the wrong idea I know you need a different pipe for the janspeed manifold the problem I'm facing though is I have janspeed manifold and decat and although I'm sure a decat will sound very loud and racecar id like a cat on the system for power and legal reasons.

Especially when the vvc apparently looses power on decat.

Question is instead of buying a sports cat from janspeed or a universal off ebay could I just cut off the old standard one and weld that on the decat pipe to get a similar effect?

Having seen a website where they tested the standard vs sports and decat on a mgf it make all of a couple of bhp difference so that logic should be the same for a zr right?

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