I did the same thing to my 07 before I installed my turbo. I dont know about the MPG, or HP losses, but it was WAY too loud. I had a magnaflow strait through muffler too. I would get head aches driving to work and on road trips. It WAS fun to be on the high way with some one right next to you with their window down and just mash it, but other than that I dont think it was worth it. It looked really good, just too dang loud.