There is often a great deal of confusion over 'back pressure' and how it is 'needed' to gain power. I don't quite know where this discussion fits, as it applies to all engine types; NA, turbo and deisel (I assume deisel anyhow!). If anyone wants to move it elsewhere then feel free!
What is back pressure?
Back pressure is (in simple terms) the resistance to the gas flow in the exhaust system. The car exhaust is designed to get the exhaust gas out of the cylinder quickly and efficiently - the more efficient the exhaust is, the more exhaust gas is removed from the cylinders leaving them with more space to fill with new unburnt air and fuel.
When designed correctly, the exhaust gas movement can actually scavenge the exhaust gas from the cylinders leaving a vacuum within the cylinder - this can mean that it is possible for the cylinder to fill with more than it's rated capacity of air, so yes, it is possible to get more air into the engine, even on NA engines!
The pipework length/diameter has a big effect on how much back pressure is present in the exhaust system (this includes the design of the exhaust manifold/header).
Increasing the diameter of the exhaust pipes (for most people, this will be through a change of the catback pipework so centre section and backbox) will result in a slower exhaust gas velocity. Reducing the gas velocity will then result in a rise in back pressure. It is this reduction in gas velocity that, at lower and mid-range rpms will actually increase back pressure and in turn reduce the power output in this rpm range. Depending on the increase in size, you could potentially lose power across the whole rpm range of the engine.
When you add a straight through catback exhaust system to your car, even if you stick to the OEM pipe diameter, you are generally removing restrictions which were present in the old system. At lower and mid-range rpms this can often have the effect of losing power low end, and upping it towards the top end of the rpm range. This is why many people find that a new catback exhaust system has left their car feeling sluggish when driving at low rpms, but feels better at the top end (where the old exhaust restrictions were limiting gas velocity and therefore limiting top end power).
What you really want is an exhaust pipe that is as narrow as possible whilst keeping back pressure as close to zero as possible - the diameter of the pipe is determined by the preferred peak power rpm range. A small diameter pipe will give higher gas velocity at lower rpm hence a power band lower down the rpm range, whereas a larger diameter pipe will give higher gas velocity at the higher rpm range, hence a power band higher up the rpm range.
The idea that back pressure is needed is completely incorrect - what you actually need is a high gas velocity and as little back pressure as possible.