There's PLENTY of example of NGRS guns with Warhead motors out there though. Out of the box the NGRS doesn't have any sort of MOSFET so I see no reason why the NGRS should be any different than any other bog standard gun to fit a brushless motor to.
Yes, it has an inbuilt speed controller that can cause issue with aftermarket electronic trigger controls.
Yes, they make loads of torque.
ANY build done wrong will wreck the internals of the gun. Brushed or brushless, it's all the same. Build according to the user requirements and within the capabilities of what you have already in there.
It's still just a motor.