It's not about back pressure. You don't want back pressure at all, it's bad. You need velocity without compromising flow.
You need to move the gas as fast as possible, not as much as possible. The exhaust system in itself is actually just there to make the car quiet and to get the gas out the back of the car. This is just to comply with legislation, nothing else. Engineers have designed the system as best they can to be quiet and flow well enough to meet legislative requirements.
The first thing people think when changing an exhaust is bigger is better, or I need to make it louder. There are lots of off the shelf systems that do this and improve performance. The performance improvement is gained by removing the restrictive exhaust. The other problem with exhausts, and this is the one that makes the biggest difference, is that you will not get an exhaust that will perform to it's maximum capabilities in all rev ranges. Fluid dynamics makes that impossible. The infinite change in velocity and flow and pulse spacings in an exhaust through all driving conditions makes tuning an exhaust impossible on a street car.
The best systems will improve mid to top end but will always sacrifice bottom end. The way to do this is have it flowing well out of the engine, then through the pipe diameter and muffler design, try not to slow it down too much before it reaches the tail pipe.
Now, after having said all that, as mentioned, I do a system that probably is one of the better velocity exhaust systems for the alloytec, it is 2 1/4" off the cats and goes through two small straight through mufflers before merging into a single 3" and out through a 3" resonator. It is nice and deep, doesn't drone, and keeps the speed of the gas going because of the merge.
As exhaust cools down, it takes up less space, so to keep it moving, you need to do one or both of two things. Reduce pipe diameter and/or merge the pulses so they aid in velocity. Larger V8 engines don't require the same tolerances in an exhaust because they have the capacity to keep the gas flowing. A small 3.6L V6 doesn't, so it needs all the help it can get.
