The problem is not the motors but the clutches that protect the gears from stripping. The motors are geared way down so they have plenty of torque (if not a lot of speed). The torque is limited by clutches so when the motion hits the stops it doesn't just shred the plastic gears. There is no position feedback like with a servo so there is no way to implement software limits.
If you want a claw I would recommend building one out of servos.