Well, I had a little time after I came home from work today and I did a little playtesting with the planetary shields. Hooray Map Editor. Put a moon on a homeworld and gave it the unique ruins to get Massive Planetary Shields very early. Then used the cheat codes to rush research and production.
First, I confirmed that the regular ones that come with the game do not block phased weapons. Then I went in and changed the shield type to phased shield generation, like what the regular ones use. That worked and blocked the phased poleron beams I was using.
Then I made ten other entries for the shield and gave them two tech requirements. Massive Planetary Shields and a level of Shields tech and increased the shield strength by 500 per level. As I advanced shield tech, a new Version of the planetary shield became available. Upgraded as I go and ran test combats. The shield rate increased.
So, the only thing the shields won't stop are troop drops and null space cannons. Now the question is, how high do I want to set them, keeping in mind that there are only 30 combat turns and things like shield regeneration are an insult to injury...
(I consider an upper limit of 20k shields to be excessive to say the least. Or maybe I should drop the number of upgrades available since any modded AI that builds these things are going to be spending 25k minerals per upgrade.)