Actually, back pressure isn't a myth.
Although it usually isn't a property that will drastically change performance in the exhaust system, it could be under the right conditions.
In a nutshell, back pressure can be said equivalent to the outlet exhaust pipe pressure, which in this case is the ambient air pressure. At sea level, P_amb ~ 96.9 kPa.
The back pressure comes into play when you are looking at the mass flow of air, and the existence of shocks. If the incoming fluid pressure of the exiting gases from the beginning of the collector happened to be say, 150kPa, using an isentropic relationship for the inlet pressure/outlet pressure with respect to the Mach number, the outlet Mach number would be much lower than the inlet. This means the mass flow is less due to shocking, which means more work required by the piston to push out the gases. This situation is obviously idealized.
In reality however, the actual inlet pressure is much much greater then the exit pressure, which in turn minimizes the chances of shocks in the flow. So it doesn't typically come into play in regular exhaust systems. The addition of a megaphone type outlet could change it though.
Because most flow properties are directly related to the was flow, length of the pipe can also be neglected, assuming the pipe is of uniform diametre .