There really isnt a gain in the butt dyno by adding them, its the ability to flow more exhaust gases and help prevent knock.

Seeing that you have a 3in downpipe, a front powerlog is what I would do then.

You said you had HPT, so run the car and see what it likes. If it has a bunch of KR, then you know you need to adjust the tune a little bit and add more exhaust flow like that powerlog. Having a wideband handy will help alot aswell.