Destroying the planet is thinking too small. Cause the star to go super nova. You destroy every resource in the system and, hopefully, ALL the enemy ships in the process. Yes, the Imperium will destroy planets but then returns for the resources later. They want to gain something but this strategy causes their crusade to be a total loss.
Otherwise you send everything you got to Terra(assuming you know where it is) and hope you can take out ONE defense platform. Might wake up the High Lords of Terra.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."