RE: Stinger length - is it important?
The backpressure comes from the pipe itself. With a tuned pipe, It works by resonating at a certain frequency. The resonation kind of causes the exhaust gasses to move back and fourth very quickly while they exit the engine. This helps draw more fuel into the engine by creating a lower pressure area just out the exhaust port of the engine in the header while the porst are open. So the length, diameter, shape, and baffles of the pipe are hgely important, but the stinger will have VERY little effect IF ANY (which I doubt) on the performance.
Just like 2 stroke dirtbike engines/pipes.