Yes, there's been lots of dyno testing on this.

For low RPM power, smaller is better. For high RPM power, larger is better. So there is not an overal optimum best. It depends on what you want.

For an A12 that makes max power at 6000 RPM (think weber carb), a 1.5" mandrel bent system is best. A14 at 6000 RPM can use a 1.75" inch, but you loose a little low-RPM power.

What goes after the muffler makes neglible difference to HP. It mostly effects the noise.