Pipe diameter and "backpressure" affect drivability, trust me. I ran open header from my house to the muffler shop (OEM usdm Type R mani) and trust me, it was LOUD and my idle was all over the place. Not to mention, every modulation of the throttle created loud burpy discontinuity in the exhaust flow. Backpressure is a good thing to the extent that you are not bottlenecking the system. If you are speaking of gas dynamics, every pipe diameter has a critical flow rate, at which point it will stop flowing. The larger the pipe, the higher the critical flow rate, the more air and therefore more fuel you can flow, leading to more power. However, exhaust scavenging is also important, and with a larger diameter pipe, you lower exh. gas velocity everywhere below the critcal flow rate, lowering the amount of available kinetic energy that effectively "pulls" on the following exhaust pulse. The larger the pipe, or in the case of the open header, the decreased mass of air flowing through the pipes, decreases the effectiveness of the scavenging, making the exhaust very ineffective in the midrange because of the lack of energy in the gas that would normally be pulling on the gas coming out behind it. Backpressure is a poor term, but as a general rule, the more continuous the gas flow, the better the exhaust will perform.