The only reason Ultron's plan would have worked in the first place was that the impact would have triggered the exotic metals located under the city. The "asteroid impact" aspect, taken by itself, just wouldn't do the job done.
Supposing the city to be approximately 2km in diameter (thanks Richard!) and given that the rock lifted appeared to be conical with a roughly 30 degree angle, that's about 10 million cubic meters, equivalent to a spherical mass approximately 1250m in diameter.
If we lift that mass to somewhere between geosynchronous orbit and the Moon's orbit (it doesn't make all that much difference exactly where in this range) and then drop it, it will hit atmosphere at about 8km/s.
The Earth Impact Effects Program can be used to estimate the outcome.
At 200km from the point of impact, the earthquake would cause "slight to moderate" damage to well-built ordinary structures. The air blast would shatter glass windows, and there might be "occasional larger fragments" of ejecta.
If you are only 100km away the outlook is more dire: the air blast would cause some types of buildings to collapse.
At 10km from the point of impact you're in for a really bad day. :-)
It isn't too unrealistic to suppose that there might be one or more large cities within 200km of the impact point, so there might be several million people in danger. But we're not talking about an extinction level event.
Taking this into consideration, I think it safe to say that dropping the city while still in the atmosphere, particularly if you blow it up first, would not be particularly dangerous to anyone who wasn't too close.