So to update on my "hissing sound" after I installed my downpipes, I had the car up on a lift twice looking for anything loose or cracked. I didn't find anything. Today I used smoke to leak check the pipes and I did not find a damn thing. I introduced the smoke through the left exhaust pipe, I found a very little leak between the cat-back and the test pipe but it was so small I could barely even notice it, eventually the smoke just exited through the right exhaust pipe but nothing else in between. I'm beginning to think it's just how it's going to sound now. I guess the pre-cats were muffling a lot of that sound.
Follow Ups
Just did a smoke leak check of my exhaust. - YlwPrlZTT20:16:08 06/12/16