What's the best stepper motor for BMG with very high extrusion rates?
A BMG extruder can be used in conjunction with the E3Dv6 hotend, the Volcano and the Super Volcano are upgrade parts for the E3Dv6 (see e3d-online), they are just differently sized heater blocks you can use to replace the normal heater block to increase the volumetric flow. In fact, it is just a v6 with a different heater block (and heater cartridge, longer nozzle threading, and often and a larger nozzle diameter). The increased heater blocks will allow for an enlarged melting zone. Although this increased melting zone, the extra length of walls will not significantly increase the resistance, most resistance comes from compressing the filament diameter to the nozzle diameter.
Note that the BMG is a similar direct extruder type as the Titan, which is produced and sold by E3D; this extruder is capable of using the Volcano upgrades using their standard pancake stepper motor (according to E3D website). You can buy these stepper motors at their website. The reason a pancake stepper is recommended is because of weight reduction (direct drive extruders carry the extruder stepper on the X-axis carriage), these extruder types gear down the speed and thus increasing the torque, a normal stepper would be too heavy and too powerful. Note that this may be a different case for 2.85 mm filament; this requires more torque anyways.