If I was an engineer working for the empire, the concept I would be pushing would be this: strip down an imperial star destroyer to basically a hyperspace engine, tractor-beam and very basic defenses, and then come up with a latching system on the front where one of these ships can latch on to an asteroid. Then you simply set a hyperspace course that brings you out of hyperspace just before hitting the planet. While in hyperspace, you de-latch from the asteroid, so that while you come out of hyperspace, the asteroid keeps going. End of life on said planet. It's a solution that's fast, cheap, mobile, and decentralized.
For all the obsessions that sci-fi shows have with lasers, if you have the ability to fling things through space at faster than the speed of light, collisions are going to be the best way to wipe out planets. Large meteors have wiped out the majority of life on earth, and they typically collide with earth at a speed somewhere around 50 miles per second. This weapon could slam a mid-sized meteor into a planet at a speed of greater than 180,000 miles per second. You could probably even engineer your own projectiles to be an extremely dense sphere for maximum impact.
