There would be 10% less money for public services, 10% fewer jobs, and we would be 10% less wealthy than we would otherwise be. The Treasury was right to share that with the British people.
As to a no-deal Brexit as a negotiating lever, it has value only if those on the other side of the negotiations believe that it is meant seriously. No one thinks that a no-deal Brexit is in the British interest, and no one believes it will influence the outcome of the negotiations.