I believe that the limiting factor for the venturi is the way they drilled the hole, with the 90-degree turn. If I'm not mistaken, the suction in a venturi comes from the pressure/velocity change that comes when water is forced to flow through a restriction (pipe ID decrease) and then rapidly accelerated (pipe ID increase). I think the Octopus uses the ID of the union as the restriction and the pump intake (which is larger than the union ID) as the expansion.
If you look at the cut-away pictures for a standard venturi, (see Kent Turbo Venturi on Marine Depot site), there is a larger contraction and expansion.
However, I would have to say that I would not be terribly unhappy if I could get 10 lpm (~600 lph) air flow. That puts it on par with the recommended air flow for the Deltec AP701, which has the same diameter.