I agree with Timmy, the magnaflow will outperform flowmaster. The magnaflow kit is nice and has mandrel bends and is stainless steel, but so is the oem dual pipes. If you can pick them up cheap somewhere like craigslist that's what I'd do if you wanted duals, then just swap a magnaflow muffler on. You could also have a shop do it but you just won't usually get mandrel bends, but it's not that huge of a concern. Also it likely won't be stainless.
Breathing in and out is good enough for me.