I don't think there is a limit on how much torque is too much when it comes to steering a truggy. The most powerful servos on the market have more than enough torque for steering. I would say no more than 200oz/in for throttle, more might cause damage, and it will make the radio tray flex. So anyway...150oz/in and up for steering, and really any reasonable amount of torque for throttle. I run a futaba S3305 servo for throttle, and it has plenty of torque (120oz/in) for brakes, and good enough speed (.20).
Good luck
Tom