Without cats will yield more power and sound. O/R pipes will come in X and H configurations and both have a distinctive sound. It will be louder, you will notice it, but it's not going to be a night and day difference. Mustangexhaust.com will have quite a few clips of exhaust sounds
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FRPP 4.10, Mac o/r X, Tri-Ax, SFC, CAI, DiabloSport tuned
Soon to add something that whistles