Hey, Rob . . .
The three main variables I played with were the pond's level, the foam spout's height above the water level and the amount of air upflowed in the main tube; minor variables were the length of the spacers and the main tube; I didn't have to play with the diameter of the foam chamber.
The foam column has to climb above the water's surface to exit; that's the critical distance. So, to increase the flow:
1. If the head of the unit is fixed, try raising the pond level to shorten the distance the foam has to travel to exit out the spout.
2. If the pond level is fixed, lower the unit's head to decrease the distance.
3. Adjusting spacers lengths can decrease the distance to be travelled.
4. Reducing the diameter of the chamber the foam rises in will allow it to climb highter.
5. Increasing the distance the air bubbles travel in the main tube allows them to attract more impurities, thereby creating thicker foam which will then be able to climb higher.
6. Increasing air flow is the simplest way to adjust -- but as you pointed out, it has limits, too.
Hopefully, one (or a combination) of these variables should solve your problem and get that sucker foaming.
Good Luck,
PS -- And, yes, I was getting a little foam under our waterfalls.