I just posed my dyno results & the only mod I have is ford racing shorty headers & hydrocarbob trap removed. I didn't do a before so i don't know how much if any HP id added. I can tell a difference in the sound and it seems to pull a little more (very little) about 300 more rpms.
I dont beleive that you will see any significant performance gains by changing out just the exhaust. Headers will, but just the piping and mufflers no. The system on the 05 Mustang is actually pretty good as it stands. Any changes that you make without headers will just change the level and tone and the sounds out the back. Flowmaster did a run and found no gains on an 05'. Please, if I am wrong someone correct me.