You have to ask yourself how much power you want to make because the exhaust won't "make" any power, but it will "support" certain power levels. If your exhaust is too big for a given power level, it will make unnecessary noise and sound farty while actually decreasing part throttle response and low end power. An exhaust system that keeps velocity up while maintaining sufficient flow to support your power goals, is the right balance to look for.
For the above mentioned reasons it's not always possible to "do it once and do it right" because your goals and needs may change over time. For example, we built an exhaust system for my son's car that performs and sounds awesome, but it is only designed to support about 300rwhp. Since he is now at that level, we are looking to go bigger. For example, his car has a cam, mildly modified heads, an MPX, and makes nearly 20psi boost. It also drives almost like stock and gets nearly 26mpg on the highway. When the blower was making 15psi this was an ideal combination but now at 20psi, the exhaust isn't big enough.
If you have fully ported heads, bigger cam, and want to make 350+rwhp, then you'll have to go bigger. If you run true duals, be sure to use a decent resonator in the middle so that the pulses are equalized or else it will sound really farty. If you go with the dual system from mandrelexhaustsystems it is available in both 2.25 and 2.5" versions. If you have an early style tank I can tell you that the pipes don't fit well, so plan on getting a late model tank if you go this route.
If having it built locally or building it yourself, I suggest using a 2 into 1 resonator and then re-splitting the exhaust behind the axle. If mandrel bent, then 2.25" twin pipes going into a 3" mid pipe is best. If you need 2.5" twin pipes due to your anticipated power levels, then you need a 3.5" in the middle to keep up with it. The popular 2.5-3.0 combo is just not a good match, particularly when the pipes are crush bent as the 3.0 " is small already and is impacted worse by the crush bends.
In my experience, glass packed resonators don't hold up well due to the heat of a supercharged engine. If possible I think it's best to use a chambered, all steel muffler in the middle and then a packing style muffler in the back. Open chamber baffled mufflers in the rear are generally very loud and raspy. A chambered muffler with packing is generally pleasing when used in the rear though they may be restrictive depending on desired power/airflow levels.