Here is a suggestion on HP vs. Exhaust Pipe size-

How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos

It looks like their suggestion would be to use a 2 1/2" Pipe, but I would take that with a grain of salt-according to their chart I should be using a 3 1/2" Pipe on my Car, but there simply isn't the room to run that size unless I jack the Car up and hang it below the Frame (ain't gonna happen )-

I went with a 3" system (850hp), and I'm actually running 4 Mufflers (2 Bullets and 2 Magnaflows), so, obviously, space must be considered in your plans as well-

I ceramic coated my Exhaust (inside & out)-

I think the sound of Mufflers is such a subjective thing, however I prefer the old "Turbo" sound (deep throated down low), so that is why I picked the Magnaflows-I think Flowmasters sound really good on a Ford engine though (not so much on a GM engine)-