I agree with those who said the car has enough power to get a load up to speed. I wouldn't be terribly concerned about the brakes, either. My worry would be trying to brake while in any sort of curve and the traction/handling issues. Part of the beauty of the Mirage is that they stripped away most of the excess weight, but that doesn't leave a huge margin of extra capacity either.
My bicycle can tow a trailer or a canoe, but it does handle differently and I wouldn't want to find myself trying to handle that load going down a mountain.