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Back pressure is utter garbage. What you want is piping small enough to maintain a high enough velocity for allow as much exhaust to exit the system as possible before it increases in density... Additionally, a well tuned system will maintain a high enough velocity to create what's know as scavenging; suction which draws more exhaust gas out of the cylinder than the piston itself has time to remove. A restriction in in a system means nothing when it comes to these factors; too big is still too big.

Of course the optimal pipe diameter depends on the dynamics of the exhaust: temperature, density, rate of thermal exchange of the conduit, humidity, length of travel, restrictions and so on. There is no one size which will be best across the board, the sizing needs to be chosen with specific operating conditions.