Right then, with the figures posted by the GM, a 10 km BB has a mass of 20125000000kg, or 20 million metric tones, and a deltav of 630 kps^2, or about .2 percent light speed, which means that in a pinch it can serve as a
0.954543499 Teraton R-bomb. If the K-ork situation gets out of hand, we might want to keep this in mind, considering how quickly we can make battleships and how long it takes to make Exterminatus weapons, although
actual Exterminatus weapons are significantly more powerful.
Edit: the effects of an explosion of this magnitude can be found here
Star Wars vs Star Trek: Nuclear Weapon Effects Calculator
+/- an order of magnitude, due to this calculator being for explosions, not kinetic strikes
Edit2: assuming a similar percentage of mass devoted to fuel as an Iowa class Battleship (17.78%), a battleship sized R-KKV would have a yield 5 times higher and a final velocity of 1% light speed. In conclusion, with the technology we currently possess, planet killing R-KKVs are impractical, however, badly damaged battleships can be used as impromptu bombardment weapons.