I have flowfasters. you have to like their style. the negatives are a drone (some may like) and the fact that they are prone to RUST (big time). I would save your $$ and buy BORLA'S (if you have friends or connections get 2 borls XS mufflers, and have them install them on your car. but do something about the resonator because it is the most restrictive thing on our cars (even moreso then the cats.) Dynomax/magnaflow make good products. I have mustang headers on mine with no cats or rez and flowmasters on the end. YOU NEED A RESONATOR!!! it is WAY to loud and sounds alittle tinny w/o a res. You could get something glass packed to go in place of the res, but my exhaust has a flowmaster y and a reducer so it istricky getting one to fit in there...
Good luck. if you have the time, knowledge, and place, I recommend going with something custom, and it will actally sound nicer with better results, but if you wanted to do that by a shop, it will cost ALOT more... if you take a look at the charts of mufflers (I think there is one regarding them on a 5.0) it has ALL the dyno results AND db ratings. Borla XS had the most HP and TQ gain and were one of the quietest. You may not wish to have quiet exhaust, BUT, if you change thecats and rez you will get a mellow tone with crazy grow like you have a sleeper ("like" you have a sleeper - hahaha sorry)
good luck with your decisions..