JC-Impreza

Currently I am running a nur-spec straight through with no cat or silencers, I have got a 3" downpipe with a sports cat (which looks like it goes straight through but does have the resonators surrounding around the outside - is this normal for how a sports cat should look?)

Main question is - Will fitting this quieten down the system? If so how much? Nur-spec is too loud for me!