The straight pipe used was routed too close to the opening in the bumper for the exhaust. Hot exhaust on the plastic bumper = melted.
What got me is the stock muffler is square, and the pipe goes in the top, and out the bottom. So there a slight drop between inlet and outlet. Will install MBRP next weekend, hopefully it will be here by then.
Time to figure out how to make it to work without making the bumper worse...