What size plumbing?

[sphelps]

Well we can dispute this forever but if you measure the pressure in the drain line (near the base) a Herbie will show a higher pressure than a Durso as no air will be in the line. A Durso may typically show close to atmospheric pressure while a Herbie will show a pressure equal to the height of the water above it. More pressure means a higher velocity and therefore more flow. How much more? Hard to say as it will totally depend on the setup. You can also eliminate most of the air from a Durso style drain if done correctly. Some people will see more dramatic results from a change over because their Durso style drain was never done correctly, while others don't notice anything.

The bottom line here however is that the main purpose of a Herbie style overflow is not more flow but rather noise and air bubble reduction. If you want more flow you should increase the pipe size or add a second drain, it really is as simple as that.