I second Hohn's post. . Banks is basically overpriced products from other vendors that they stamp BANKS on. You should be ok with what you have. . If you are running a stock 35, a 4" DP will help a little bit, but it is when you step up to the plate to get a bigger turbo that a 4" downpipe is beneficial. Just thought I would chime in. But just steer clear of BANKS junk. FYI, They have Poor, POOR customer service and support too. .
-Chris-