well there's the magnaflow catback and then there's a custom magnaflow exhaust with just the muffler. the latter needing to be welded on of course, and being louder than the former, but probably more hp producing since it is much more straight-through. I have a custom magnaflow with the street series stainless steel round muffler. I can't give you any numbers but I can tell you it's like running with no muffler only ten times quieter. I would just go no muffler since that's the cheapest way to go, but one night while testing my car with a couple of friends, the police sent the chopper after us since we must have woken up the whole block.
Good thing we had just finished b/c just when we sat down in the dark, this heli was shining a spot light on every street in the area.
Let's just say I'm happy to have my magnaflow high-flow cat with magnaflow muffler...for sound and Performance reasons.
The setup I have might be considered too loud to some anal cops, so I also have an Arospeed Exhaust Control Valve to make it quiet when I want, and loud and fast when I want.