Are you doing a full exhaust (headers back)? That will help you most with flow. However your BEST bet if your only going cat back is as follows:
mandrel bend 2.5'' piping
Yes, I said no muffler. As opposed to what most people thing, most of your restriction is in the muffler.
Notice all the baffles? Thats where your exhaust gets thrown around causing for back-pressure.
Of if you don't want to go with no muffler, you can always go with a "bullet," muffler (as shown below):
Hope this helps!