my buddy has a black 10th ann cobra, he put straight pipes on it...sounded crazy....but A: backfires WAY TOO much, B: way too loud for other people, people living around you and cops are going to harass you like nobodys business.
I've got strait pipes with the stock exhaust piping....I'm going to get a MAC one of these days, but the strait pipes sound pretty good.....really loud under WOT but still pretty quiet at normal cruising speeds. It does backfire quite a bit under deceleration but it sounds cool.
I don't see how a F.I. car would make more power with mufflers when it's not dependant on back pressure. I realize that having no mufflers is somewhat illegal in a lot of areas, but a N/A motor vs. a F.I. motor are two completely different things. I took the mufflers off of mine this weekend, and I love the sound, had a few compliments, and it sounds mean as hell.