I dunno what Mag comes with the Roush (all custom for Roush, or just custom settings with an 'x' model muffler), but mine sound fantastic! I had Flowmasters on before and was worried the Mag's wouldn't sound anywhere near as good. They're not quite as loud at low rev's (Flow's sounded FAR better at idle), but have a better depth and get plenty loud enough once you hit the gas.
Flowmaster's added nice aggressive volume, the Magnaflow's actually make the car sound bigger, not just louder.