On the e3d forums they've said that the theoretical maximum on the volcano is 40 cubic mm/s and the v6 is 15 cubic mm/s.
I have managed to get close to that maximum flow rate on my volcano with a nimble extruder.
1mm nozzle: 1.2mm extrusion width x 0.75mm layers at 30mm/s = 27 cubic mm/s
I attempted printing at 50mm/s but was stalling out the extruder on fast print moves as the volume extruded was 45 cubic mm/s at those speeds. I didn't spend much more time finding the absolute maximum but could possibly do so. (44 mm/s would put it just below 40 cubic mm/s)
I am using a 24v 30w heater in my volcano, using typical temperatures for pla (200°c), same as on my v6, volcano nozzle temperatures remained consistent even though the extruder was stalling, so I don't believe a higher wattage heater cartridge will improve volumetric output. As Tony said wattage is more a function of maximum temperature.
I have not yet attempted to test if increasing the nozzle temperature impacts the maximum volumetric output as I assumed it would have a negative impact on areas where the speeds are low enough that the volumetric output is within the limit of the volcano.