Cutting out a section of exhaust shouldn't really change back pressure as long as the remaining pipe diameter doesn't change, and in my mind shorter pipes could only decrease back pressure since there's a shorter restricted environment it has to travel through before getting to atmosphere. Which kind of explains classic side pipes by itself.
I kind of cheated and took a refresher course in back pressure by googling it, but backpressure is only a function of overall flow vs. velocity, and I'll credit Iminhell for making a similar argument against larger throttle bodies. Smaller diameter pipes will move exhaust at faster velocities with less CFMs of air movement, and larger diameter pipes can only move air at fast velocities with substantially more CFM exhaust flow. So it basically comes down to where in the powerband an engine is making peak HP and producing exhaust flow.
There's a ton of physics behind exhaust flow and I don't pretend to know much of it, but I think what little amount of backpressure is designed into exhaust flow has more to do with synchronising exhaust gas pulses and making a smoother running engine.