This immunity isn't mentioned in the DR guide... Though, while it doesn't say that armour-scale SPs are different from personal-scale SPs, it's hard to explain how armour-scale units can take 5 hits from weapons of the same rank while personal-scale units can only take one, otherwise. Mind you, it doesn't actually say that personal scale units can only take one hit from weapons of the same rank, either. It just isn't clear.
4 or 5 SP seems like enough for this pack, to me. The GM determines what effects weapons will have on the pack at the personal scale, giving it higher SP just tells the GM not to let it be disabled with one shot, even while the shields are down, and even with a very powerful weapon, unless the weapon used is armour-scale. This only seems useful if a design goal is being able to recover still-functional packs from dead soldiers...
I'm not going to say anything else on the subject, it's clear to me that significant changes to the DR rules need to be made before they can be used as a stable basis for arguments about what the numbers mean.