Even if the motor is handout, there are still variances in the build quality. At some events, you are able to purchase additional handout motors. If that option is available, I would be the first one to buy a few hand out motors at the event and dyno them to get the best one. I did this before even back in the TCS 540 silver can days to win the Canadian TCS.
If the club allows a $50 hobbywing to be run. I would buy like 10 and dyno them as well and sell off the rest on ebay. You can stop racers from doing this. No matter even if you spec a motor, there are always differences in tolerances.
One good way to prevent this is to have a club have 10 handout motors (timing locked and sealed). Drivers pick a number and thats the motor they are given for that round. After the round, it is collected, and numbers are picked again for the next heat and so forth, going into the finals. So no driver has control which motor they get, and it is will make racing a lot more fair. Problem is the time to organize, allow drivers time to pick and install the motor and return the motor after a round. But in the end, it becomes cheaper for everyone to participate in that class. Perhaps an additional fee of $5 could be added to offset the cost of the club buying 10 spec motors for USGT. IF there are multiple heats of USGT, you can stagger the heats in between mod, stock to allow drivers to shuffle the motors around. Just an idea.