Well, there are several 'primitive' Shields that work like cover (i. e. Tower Shield - IH) though the Sinford is not one of them. It is made of ceramite and therefore is just as reliable as Carapace Armor or any other Weapon that gets used to parry stuff. If this shield would suffer from the Cover Rules all your armor should because that is simply what it is: Stackable Armor. The problem of this shield, that makes it a formidable upgrade, is its fireing port that makes it an awe-inspiring tool in combination with a good shotgun. (Though I wonder why a cleric is using such a dedicated arbites shield^^)
So rule-wise the shield does not have the cover rules and therefore does not get damaged by every penetrating hit. (That would be pathetic for 4 AP… even an unarmed attack could destroy the arbites most resilient tactical shield that is used to protect from small-arms fire and even explosives)
Well hold on, are you saying that plasteel is weaker then ceramite? Because if it is then I can understand if it does suffer from the cover rules, but if plasteel is equivalent or better then ceramite then it should be subject to the same rules as Synford-Pattern Lockshield.
Even though the Navel Shield uses the cover rules it is still plasteel and for cover rules it should have a 32 AP but since it's a shield instead has a 8 AP. So I say that if your going to apply cover rules to a plasteel shield then I argue that you should have to beat a 32 AP first before you reduce the 8 AP to 7 AP so on and so forth.
Otherwise make it a like how the Synford-Pattern Lockshield is and make it Stackable as well. Because quoting you it would be pathetic that even a well placed unarmed attack could destroyed a Navel Shield made from plasteel.