For Modeller: Yes, a lot of forest was flattened by the Tunguska airburst, but it is not usually counted as an impact, because there was no crater.
According to
http://www.passc.net/EarthImpactDatabase, our last three known impact events have been (1) Wabar, in the Empty Quarter of Arabia, about 1870, crater diameter 110 metres; (2) Sikhote Alin, at the northern end of the Sea of Japan, February 1947, crater diameter 20 metres; and (3) Carancas, southern end of Peru, September 2007, crater diameter 14 metres.
Even if an asteroid made a big hole into the mantle, lava wouldn't come pouring out, because it isn't under any pressure there except its own weight. It would just ooze out until it solidified. As for Knuckledragger's 'hole right through the earth', quite apart from the energy needed to create it, where on earth (or rather whereabouts off the earth) would the material from the hole have gone? I think that if the asteroid was too big and/or too fast to be slowed down, its impact would be generating enough heat to melt it, so it would lose its penetrative power that way instead.