What kind of engine management are you running? Do you plan on getting a catted or catless downpipe? People think because of the taper, the Cobb DP is going to be more restrictive. That is a misconception. You won't gain much if anything at all by switching to a full 3" DP campared to your Cobb DP. So to answer your question, generally no. You won't need to retune your car. Unless you run into boost creep/boost spike issues from running catless, your current tune should be fine (assuming you are actually tuned for the DP).