@GM: "Actually that doesn't seem to take into account the amount of mass that would be lost to gravitational friction."
Essentially zero, forget it.
"...That would depend on the angle that the object entered the atmospher along with it's velocity..."
At a relative velocity of over 20km/s, as long as the entire object strikes the earth, the angle makes almost no difference. The kinetic energy transfer is over 90% and the momentum transfer is well over 50%. The object becomes effectively plasma and generates a succession of pressure waves that propagate inward and along the surface of the the Earth, with a small proportion of ejecta.
"...and more importantly its composition."
That will be pretty well known at flyby.
"As indeed would it's velocity at impact."
Yes, I heard over 20km/s.
"Since they're not quite sure what it's made of nobody can say exactly what it's mass is to start with."
Yes, but most asteroids fall within a moderate density range. Since this has no jets and is small, it's not a pile of rocks but probably a big rock, i.e. reasonably dense. Certainly moreso than water, so its mass should exceed 25 megatons for r > 300m.
So there's no way anybody could predict what the damage would be were it to hit the earth.
"...Even if it were to hit the earth it's obviously more likely that it would land at sea than on land."
If by sea, worse (waves)!