There is a “speedbooster” accessory on the market that can be added between the motor and power supply. Does GB offer something similar?

No, we do not offer a similar style unit to this.  The design of this unit increases the voltage coming into the motor to increase its capacity.  Our motors are only rated for 120V, so increasing the voltage coming into the motor will damage it.    Any GB puller used with this type of accessory will have any warranty voided, and GB is not responsible for any damage done to the puller or any associated equipment used with the puller.