Gti rally car

okay guys I have this current rally car in my pic wanted to no would I have more low down power with cat in currently cat removed but fonnd with the janspeed back box it’s very raspy and pops and bangs on deceleration I had a cat converter that come with janspeed exhaust looks totally different to the standard cat so don’t know if they do a performance cat that perhaps this is can anyone help please
 
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