I have a set of Flowmaster Super44's for my 2004 GT.
My car has a MAC cat-back and a MAC prochamber midpipe as well.
I love the flowmasters and am comfortable with the inside riding noise.
They have a great bubbling/burbling sound at idle and seem to sound deepest around 2100rpms.
Gotta let them mature though after putting them on, they get so much deeper as you drive them.
I would recommend them to anyone though for the sound and the price.