So how much damage would a 70 mile asteroid actually do?

It wouldn't wipe the world immediately but most life probably wouldn't survive. First all the aerosols kicked up from the impact would get into the stratosphere where they would have a residence time of a couple years. Eventually the aerosols would spread in the stratosphere covering the globe blocking most of the suns solar radiation causing nuclear winter. This nuclear winter is what would kill most life. Nuclear winter is possible with 5km diameter comet or asteroid, so 70miles would absolutely wipe us out.

Yes.

Massive fireball, shock wave at hundreds of miles per hour, vaporized dozens of feet of ocean, rain of molten rock and/ or tsunami maybe half a mile or more high, incredibly cold winter following lasting years or decades.

In short, virtually all non-microbe land life on the planet extinguished, and probably most of the ocean life as well. Pretty much back to bacteria worldwide, or close to it.

The asteroid in the movie is roughly 1,000 times BIGGER than the one that destroyed 3/4ths of the world's species 65 million years ago, including wiping out the dinosaurs.

On the bright side, we haven't been hit with a 70-mile-wide asteroid for billions of years, and aren't remotely likely to be.

Pretty much.

It wouldn't be close cracking the planet in half or some asinine stuff like that, but the energy released by the collision would be more than enough to sterilize the surface