hmmmm .. that stock cat sounded effy, your test pipe should help , maybe clear up some other problem to help dial in on the bigger one. this what i did on both of my s4's went over the grounds/ connections, cleaned up and redid some. also had my vaccum system checked . i was to lazy to so i got my friend to do it . hmmm how do your plugs look? white and ashy or burnt thick black? if yours is a s5 wasnt there a funky fuel set up recall? i might be smoking crack.

 

I just changed out the cat whoa what i difference. can you snap a few pics to show me what grounds? also just changed out plugs to laser cut iridium's, but they looked ok for being the ORIGINAL PLUGS! omg couldn't believe it. recall? hmm dunno is it true you can do the recall at any time?