Well, it really comes down to the height. I think its plainly obvious that without a pressure assisted air input, you are looking at a pump that is much larger than a dart to get 3000+lph if its pumping against 3-4' of water pressure.
I noticed that all the venturi inserts were in the 1.3 to 1.4 inch range. This is odd to me, because for the flow of the pump, I would think that a smaller venturi would be better for more air. On the Laguna based pumps, the ones with similar flow/wattage use a 1" or even 7/8" diameter venturi. Now, this isnt 'talking crap' or something, as I suspect that the dart might be slightly different, or that restricting it that much might cause it to choke/overheat... but from seeing how Scott's venturis were assembled, I really wonder what his understanding of a venturi might be, esp since the gate valve is used in many cases to 'tweak' the intake... which means that there is significant headroom for venturi improvement.
That being said, I still dont see this motor pulling much more than say, 2000lph (if that) even with a good venturi simply due to the height of the skimmer. I think its safe to assume that ALL Volcanoes, unless 3' and under, are better off with a linear air pump.
Another method could be to swap out the bodies for a cone. I wonder why Spazz hasn't tried making a full cone like the ATB/KZ's, since his current 'half-pipe' methods to create a cylinder, as well as the transition would be replaced by one 3-4' tall cone. A cone, besides its other claimed benefits, would allow for the 4' tall Volcano to run with a waterline about 12" lower than it is right now. I have noticed that on many Volcanoes, there is a flange on the body right around the bubble plate. Simply making a cone body that could be swapped out for the current cylinder + reducer would be simple. Maybe some of you current owners would like to try it. You just have to have the twist lock and flange matched up. Making this cone body would be simpler for Spazz too since it eliminates having to make the reducer all together (you can look at it as him just having to make the two 'half-pipes', but it would actually mean less material than the two half-pipes due to the cone's taper... like so:
Just an idea... but I think it might work well. Currently, Tom's waterline is up in the neck... about where the #1 line is. With a cone body, the waterline could be lowered a bit since the midpoint of a cone is much lower than a cylinder... dont know how much... but this would also take the back pressure off the motor as well, so who knows how much lower it could go... maybe lower than the #2 line depending on how much air the pump can then take in. There may even be more than one 'sweet spot' as in, as the waterine goes down, the air intake continues to rise so that you could literally have a sweet spot where the waterline is at the top with less airflow, and then a sweet spot everywhere in between to a waterline that is only a foot or so above the bubble plate but the resulting air increase would still be pushing bubbles into the neck.
Just an idea...