I have a DC 4-2-1 header with a Type-R cat-back... It gives good mid and high range power, but a 4-1 header and a larger diameter exhaust should have a better top end and not as good of a mid range. Increasing your the size of your exhaust piping will get rid of more back pressure, but it will also lower the exhaust velocity.

Back pressure is when the engine has more exhaust than can normally flow through the exhaust pipes; the engine tries to push the exhaust out, but when there is back pressure, it pushes back against the engine and wastes energy (it slows the piston). There may also be exhaust gases that are left inside the combustion chamber, which doesn't combust well (since it already combusted) and makes less space for the intake air / fuel.

Increasing the capacity of the exhaust system is the obvious solution to backpressure, but you have to consider exhaust velocity as well. When exhaust gasses are flowing, they create an area of low pressure behind them. That area of low pressure is a vacuum, and the faster the exhaust gasses flow, the more vacuum that is created. That vacuum is basically the opposite of back pressure; instead of resisting the engine, it helps it. It sucks remaining exhaust out of the engine, and helps suck some intake in. Then the exhaust valves close and the piston does the rest of the work sucking in the air / fuel. This is not just some bonus phenominon, engines depend on it. Some people will say that engines need some back pressure, but that is not true; what the engine really needs is exhaust velocity.

When you increase the size of the exhaust piping, it lowers the pressure of the exhaust gases. And lower pressure means lower velocity. There is only one level of exhaust flow that will have ideal exhaust velocity, so that's why you choose your setup depending on where you want your power.

I believe the actual power gains come from lowering the ratio of back pressure to exhaust velocity. If a piece flows better than stock, then it helps the exhaust get out faster, which means less time for back pressure to build up and better velocity as well.

That's just how I understand it anyways...