Sorry wickedfast. I blame lack of coffee! (Look at that I'm apologizing...maybe someone secretly replaced my mazda 6 with an evo IX....)
You have a 6s. Your exhaust looks like this.
A few blinks and you'll see the problem. The japanese engineers figured they'd have a good laugh at the US's expense. There are more bends and twists in the first few feet of our exhaust then turns in the nuerburgring! (Ok thats exaggerating slightly! Regardless most people get sick just looking at that).
In this type of situation, modifying the catback or mufflers will provide little or no gain, because all of the restriction is further upstream. This is verified by dyno's, as no one besides the various exhaust manufacturers have shown any gain whatsoever with any of the available catbacks on the market. (on owner dynos).
If anything, some of the catbacks are actually reducing overall power, because they're slowing down what little exhaust velocity there is, by the time it gets done going through the twists and honeycombs of doom.
Now if you were to replace the exhaust manifolds and the restrictive (and dangerous) pre-cats with the wagner or lovely CP-E manifolds, then you could start working on the exhaust velocity of the rest of the system. This is assuming that the catback is even a restriction to begin with. Its a single pipe, with two mufflers handling the exhaust streams. Not exactly restrictive. Wagner saw zero gain on the dyno on the 6i headers/ypipe and with/without the stock catback or wagner catback. I'd say if anything the catback section of the car is actually fairly well designed in terms of bends and restrictions.
Picking an exhaust size for a particular engine...tough. Two ways to do it. Wait till a bunch of other people dyno their car with tons of different exhaust sizes, write down the info, and then purchase the best setup for your application. (http://www.contour.org
Or... Calculate the maximum volume displacement at the particular rpm you are making maximum power at. Match the exhaust size up to a similar volumetric displacement. The higher the rpm, the less velocity you'll have at lower rpms, which will result in a loss of low end power. Conversely you can setup the exhaust for max flow at your torque peak, and get a completely different effect.
Toadster, the comment on the RWD vehicles was referring to V style configurations. Aka V6, 8, 12, 24, 32, 128. Etc