Actually such an impact would be much worst. First of all the object is about the size of the moon (the shadow on the Earth was nearly equal to the whole of North American. At impact it would start to turn into a plasma as it does in the clip But the blast wave would streadout faster then the heat wave. The force of the blast wave would strip bare the Earths surface, flatinging all man's structures nearly level with the ground.
**AS the object hits it buring itself through to the Earth's core, the oceans would immediately flash boil, quickly breaking down the water into hydrogena and oxegen to be ignited into an atomsphreic plasma, The whole of the atmosphere would be fire, plasma, and sparks (the size of small cars to the size of large buildings. For thousands of years the whole of the Earth would be molten--the gases having been blown into space (the extreme heat would excite the gases and they would expand beyond the radius of the Earth's gravity to hold onto it. As the Earth started to cool--ash and other gases would reconstitute the Earth's atmosphere making Venus seem like a paradise to what the Earth would be.