If I were you I would be VERY careful and remove and SAVE the factory muffs, preferably keeping the entire factory catback uncut. Those pipes are bitchin' and will soon be legendary. I say again TRY TO SAVE THEM UNMOLESTED AND COMPLETELY INTACT, Even if it means pulling the rearend to do it. In 10 years you will thank me.
Exhaust noise is sorta like power...you get used to both entirely too easily. I got the Magnaflows and love 'em (but the hooligan in me wants more "f-you, anarchy, loud-pipes-define-lives" good ol' fashioned rackit!). I've listened to the clips on mustang exhaust and think there is a good chance Magnaflows won't be too much louder that factory bullitts. If you're itchin' to be rowdy, I would suggest the 'packs even though they are not my style.