400 meters cubed is 6.4x10^7 cubic meters of stuff. If we assume an average density similar to ice, then that will be around 6.4x10^10 kg. If it impacts at earth's escape velocity (11,200 m/s) it will release 6.4x10^10 * (11200 ^ 2) = 8x10^18 joules of energy = about 2000 megatons.
Above is all seat-of-the-pants guestimates.
