I have a few SEIOs, the 620, 820, 1100, and a 1500. I plug them in SxS with the MJ mods (or my aquaclear mods) and get an idea based on that. Usually the differences are large enough to easily tell...or at least give an estimate something like 'this Aquaclear 10 with a nikko 3-blade prop does as least as much flow as a SEIO 620, but not quite an 820, and with only 3.3 watts according to my meter'. The garbage bag test would be nice, but that would require the space which last I checked...my tanks dont have. The garbage bag test is good however, if done multiple times over a long enough time sample. There is no 'head-pressure' as both sides of the bag are submerged.
The only way I have thought of being accurate about testing these stream pumps would be by measuring the velocity of the output x the area of the output. Using an underwater version of what meteorologists use to measure wind speed, then calculating the are of the output nozzle, we could have a calculation.
As for being able to outperform a SEIO, sure. Tunze, maybe a bit harder, but still capable. Notice how Tunzes move more with even less wattage than a SEIO. Thats because SEIOs dont use a true propeller design, Tunzes do. SEIOs use a hybrid corkscrew/impeller design. Its so that fish cant swim in the motor if they should ever turn off. With the Tunzes, the idea is that they never fully turn off...you simply turn down their flow enough to leave them running yet the flow goes down to 30% thanks to their DC motors. SEIOs have 4-pole AC motors.
Hey, if I can outperform a SEIO 620 with a Aquaclear 10 (4.5 watts normally) and a cheap 3-blade racing prop in a 10 second mod...the sky is the limit.
I put dual 2.3" props on a Aquaclear 5000/901/110 series, and the results were up there with a Tunze 6200...maybe more. After I put dual 3" props to put on there...I bet I can outdo the Tunze at the same 55 watts.