Hmm...I don't know what math is being used, but there's a lot more involved with destroying things on the surface than people realize, and speed is only part of the equation (I mean that literately, there is an equation, several to be honest). You are better off using ships to glass the planet for one very important reason: if two meteors with a diameter of about 150 km were to strike opposite sides of the Earth today, it would completely change the rotation of the planet, and kill off everything alive, including bacteria.
I like the discussion, I love that we're making a story, but please reconsider this idea of destroying everything on the planet. If you want to glass it, glass it; don't throw sh*t at it. The Chicxulub crater was created by the meteor that killed the dinosaurs, a meteor with a diameter of 180 km. It changed the atmosphere, not to mention everything else. Having twenty-five meteors with...what, 500x500x500, volume? I'm hoping that's in meters, because that would otherwise mean you're sending the new species killer that's bigger than what killed the dinosaurs >.> Heck with the species, the planet itself would be knocked from its orbit. If it's in meters, which it should for the sake of scale, their combined radius wouldn't even be a kilometer. They will burn up in the atmosphere long enough that when they hit, they will not do very much damage at all because they've lost a portion of their speed due to air friction, if not just burn up out right. You'd knock down maybe one city, and it'd have to be reasonably small.
Please, if you must, just glass it, because while I can talk all day about how easy it would be to send a meteor to f*ck up a planet out of its orbit, the math of figuring out how to "stably" not destroy a planet would require enough calculations to where NPCs will ask why.