I really dont like this idea. Why? Well, having a Hyper velocity gun on a planet (HVG) for example, really made great use of the rebel raid tactic here. You could send in a few squads of Airspeeders, take out the guns and then send your fleet over to attack the planet.
I feel that this 'after thought' was brought in because Imperial fans were struggling against the Rebels and their ION cannon because they cant raid. But the Empire is more about using brute force - so the small advantage the rebels have, has been wiped out in terms of ground to space defence.
If it was creating problems, then limit each side to just 1 cannon/gun because a fleet of Star Destroyers will make toast of a HP.