Back pressure is bad. You don't want back pressure.
To quote someone from another forum:
The goal of the exhaust is to get the exhaust out as efficiently as possible, and to quiet it down. The headers affect torque/power directly and greatly, those must be chosen specifically for the engine and rpm usage. But from the point past the collectors where scavenging is done(at the ideal crossover point), past that any restriction costs power, period.
From that point on, the exhaust does nothing to gain power, nothing, it can only take away power due to restrictions. The catalytic converters are restrictions, as are the mufflers, the bends, and the tail pipes. A straight length of pipe past the crossover point produces less power than if the exhaust ends right there. Any combination of big pipes or small pipes after the crossover point hurts power.
The best power is had from reducing restrictions beyond the ideal crossover point. Don't confuse where you have your crossover or the factory has it with the real ideal point. Racers know how to find that point(run it WOT through the traps, shut off and find the hot spot after the collectors), but the location is behind the collectors very close, say 6-20". It isn't where it's convenient to place a crossover pipe.
So, open up the exhaust past the collectors as much as you can, and select mufflers that you like for their sound.
We can argue about what size pipes are not worth the money or too much trouble to install etc. But do not suggest that an engine makes more power with a smaller pipe size after the collectors. That is BS, wrong, and a myth. Don't quote loss of power after big exhaust changes, every single example of that is the failure of the owner to richen up the mixture, the A/F ratio.
When you open up the exhaust, that leans the mixture, and the required action is to add more fuel(which is more power). The answer is not to install smaller pipes, that richens the mixture and returns the power to the lower potential from before. With bigger pipes, you can add more fuel, and yes that means altering the PCM programming, which isn't always easy. But the point is the same, bigger pipes create less restriction, allowing the gases to get out easier. The goal is not to make it hard for the exhaust to get out(so it's faster), the goal is to get it out of the way of the gases coming out of the collectors next."