I think that the biggest threat would be from the more likely impact site, the ocean. Even a small, dense object traveling at these speeds hitting the ocean could take out more cities than just one. Could take out entire contries if it hit the right spot.

We definately need a new system to detect the smaller rocks further out. I remember in the aftermath of Shoemaker-Levy there was much talk about building one, but as always when the images were no longer dominating the TV the talk died down. I hope the next wake-up call, when it comes, isn't too nasty.

Perhaps, perhaps not, it would weigh in at something around 500,000 to 1,000,000 tonnes. Depending on the impact velocity, you are talking a couple of megatons. A T-object is in the Gigaton or Tetraton range. A tsunami might result from a small object impacting off a coastal city but it wouldn't be widespread. Not enough energy involved.

A T-object however......
Thats bend over and kiss your...... goodbye/global extinction event time. For the whole planet.