If the starship derives its energy from its store of antimatter, it doesn't matter how fast the ship is moving when it hits a planet, just so long as it hits the planet with sufficient force to rupture its antimatter containment system. There is no free energy derived from going to warp, some antimatter is consumed to achieve warp speed so there is less antimatter and more warp energy, but the two balance each other out.
According to predictions based on relativistic equations, objects going faster at FTL speeds have actually less energy than those going just above the speed of light. An object as rest has the same kinetic energy as an object traveling at infinite speed. Warp drive is just a way of dividing up that FTL speed. Warp 10 is the equivalent of an object at rest energy-wise.