Besides impacting on hull, have we talked about impacting on planets?
For a small 1kg ball of generic metal, 1000km/s relative is 0.5TJ, which I would expect to generate a nuke-like explosion in the upper atmosphere.
But as the speeds increase and the chunk gets heavier (mass driver is 1kg, fragment missile might be 1-10kg, kinetic strike missile could be 10tons), you could get some obscene energies, and then what happens?
10 tons (15tons full load) at 10 000 km/s relative (say a missile burning all delta-v, tech based off geosurvey vessel) is 500PJ. Even if it turns into plasma in the upper atmosphere, that's alot of air heading downwards very fast.
Besides, a long-range anti-planet missile could be fine-tuned for maximum energy yield with a really long burn time (high efficiency engines).