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.

The cable pullers are rated for 10,000lbs and 8000lbs. Is the the constant working load?

No,  These ratings are the maximum rating for each of these respective units.

For the CP10K puller, we list the pull strength as 10,000lbs.  This is the maximum force the puller can pull for a short amount of time, normally 5-7 minutes, before the unit should be shut down to let the motor cool.  For continuous pulling, this unit can pull 8,000lbs at a consistant rate without overheating the motor.

For the CP800 and CP8000 series pullers, the maximum pull strength is 8,000lbs with 4,000lbs as the continuous pull for.  Same thing as the CP10K.  If the unit is being ran at full strength, then it should only be ran for a short time per pull and allow the motor to cool.