I'm pretty sure the golden chest pulls out of the same item pool that the raid bosses do(the Warrior, Terramorphus, Vermivorous). It might always be purple stuff, but a lot of the purple stuff those guys drop is garbage.
I've only used one key(only had one), it spit out two level 50 shields with identical max shield capacity to a purple shield I'd gotten off the Warrior(which I actually later found one from him that was over 50k).
I think the key(s) were meant more as a "you just hit level 50 and your gear sucks, here's a shot at something to get you the rest of the way through the story". I know I regret not using it sooner, would have helped a lot.
I've only used one key(only had one), it spit out two level 50 shields with identical max shield capacity to a purple shield I'd gotten off the Warrior(which I actually later found one from him that was over 50k).
I think the key(s) were meant more as a "you just hit level 50 and your gear sucks, here's a shot at something to get you the rest of the way through the story". I know I regret not using it sooner, would have helped a lot.