[quacked]

30*150 = 4500 lb. There are cases of planes crashing because everyone rushed to the front or the back to avoid smoke.

I know it's not the same as a commercial airliner, but I used to teach hang gliding on the beach and we'd sometimes get a little packed sand in the back of the keel (which is the name for the central bar that the wings were mirrored across). The total weight of the student plus the glider could get up to ~280 lb, and the amount of packed sand would be less than a pound or two, but the flight characteristics of the gliders would change drastically, to the point where we could barely get them in the air.

As another commenter notes below, the few people moving around the cabin is less about control ability and safety than it is about fuel efficiency.