But you're not going to meaningfully spread the energy over time. If the bits are going to hit that means they didn't move more than 4000km after the strike.

But you're not going to meaningfully spread the energy over time. If the bits are going to hit that means they didn't move more than 4000km after the strike. Assuming they're distributed in a sphere that means none hits more than 4000km early, either (and likewise 4000km late)--and since we are talking about objects with a relative velocity of a few km/sec that means it's dispersed over at most a couple of hours. By the back of the envelope you're looking at about a megasun. Once again, a scenario where letting the rock just hit is not as bad--energy spent gouging a huge crater is energy not spend blasting civilization.