In the Philip K. Dick short story The Variable Man, Earth is at war with the Centaurian Empire (which in the story completely circles our solar system). Earth scientists invent a missile that will fly toward Centaurus at FTL speeds, and then drop out of FTL inside the star. Due to the method of FTL, dropping out of FTL speeds means that the object has nearly infinite mass at relativistic speeds, which would cause the sun to implode.
It's not "warp", but it is FTL. When traveling at near FTL speeds, according to relativity the object will have nearly infinite mass. That would be the cause of any damage. Warp bubbles work differently, though, in that the mass of the object at warp is actually decreased.
I think we should try to maintain conservation of energy, if it takes a certain amount of energy to achieve that warp velocity, then no more than that amount of energy will be released upon impact with a planet, so it makes no difference whether the ship explodes on the surface due to a breach of its antimatter containment or if it impacts on the surface at maximum warp velocity, and any antimatter left unused would then react with the ship's matter hull and with the planet's matter surface to achieve a total conversion of mass to energy, that would be a big explosion, a matter/antimatter explosion is 100 times more powerful per unit mass than a thermonuclear bomb.