Rover owners: Emptying a Cat.......

I have just purchased a back box that came with a cat attached. I removed (by angle grinder) the cat, it rattles a little. so, to empty the contents I assume its a decat, 'of sorts' at least.

The only difference I can see between a straight decat pipe and the empty cat is the shape and volume. Is there anything to be gained in adding the proper decat?

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