Curt,

I believe it was determined that the duration of the blow-down period, is more consequential to the ability of the engine to make a gain from a tuned pipe, than is simply the exhaust timing.

The reason a long exhaust duration works well is because if the intake bypass duration is not also lengthened, the blow-down period is.

This gives the pipe enough time to cram more of the fresh mixture, back into the cylinder, without this extra charge escaping back to the crankcase. A longer duration exhaust is necessary for breathing, at high RPM.

If a pipe is intended to boost engine power to 16,000 RPM and northward, you do need 160 degrees and more exhaust duration.

For high boost at 9,000 RPM, 145-150 degrees can be very usable, given the intake bypass period is short enough for a usable blow-down.

The pipe that you need for this is longer, of course.