Why not use two drills to add more even torque (just a thought it's been some time since my physics class in high school)? You could even have them controlled by a microcontroller, with a switch on the trigger for easier control.
You could also perhaps get more powerful electric motors mounted more permanently. Maybe the electric motors that hybrid vehicles use? Or just electric high torque motors that are powerful enough to increase the speed.
He's showing off what he made, I don't think he ever asked for suggestions for improving. It seems every time he's post a weapon, there's a dozen people that come on here with ideas on how it could be improved. I could understand if he'd asked for help, but can we just enjoy what he's made?