At the speed it would hit, it doesn't really matter what it is made of, just the total weight. Roughly, it would wipe out the continent it hit, cause coastal devastation all over the planet, and darken the skies for decades, causing freezing winters etc. Probably more than 90% of the population would die. It is, however, very, very improbable.

Depends on its speed. I can take a bullet slug (the lead part) and throw it at you it will bounce off harmlessly. I put that same slug in its cartridge with gunpowder and shoot it out of a gun it is like to hurt a lot if not kill you. The difference is the velocity. I read some time ago that it is thought that the Tunguska Event was a 'slow' comet that was traveling slow enough to where instead of punching all the way through the atmosphere to make an impact crater it had time enough to reach a critical amount of heat (due to friction) and 'explode' well before hitting the ground. If it had been going fast enough it would not have had time to reach its critical heat, thus would have plowed into the ground and detonate there. Actually had it done that it might have done less damage most of the energy would have been absorbed in the ground, not radiated over such a wide distance. How the Comet is 'packed' would also have some effect. Today we are thinking that comets come in 'solid' forms a single mass or a collection of large chunks and 'soft' forms, a collection of smaller loosely packed ice and debris. This is like the difference between throwing a chunk of ice at a person and a packed snow ball. A chunk of ice would retain its shape longer when it punches through the atmosphere, a lose 'snowball' would break up. Hitting the atmosphere at a great velocity is like hitting a wall. It is similar to the difference of jumping off a diving board and jumping off the Golden Gate Bridge. Water is 'soft' if your speed is not to great but becomes 'hard' when you have a greater velocity. All of these factors combined means there is a wide scale of potential consequences. On one extreme with a hard packed comet traveling at the right rate of speed (not to slow, not to fast) it would punch through the atmosphere and dig a crater. At the other end with a fast moving 'snowball' it could 'smash' in the higher atmosphere, 'explode' (smash apart) producing a shock wave. How strong that shock wave is depends on its size, speed and how far up it 'breaks apart'. If its high enough, slow enough and 'soft' enough the shock wave could (in theory) be no worse than the most powerful sonic boom we know of at ground level. However it could also be that the conditions are correct that it is like detonating a thermonuclear device a few miles above the ground - you get a massive shock wave over a greater distance.

All the dinosaurs would die.