1. There IS no SAAMI standard for .380 +P. There are OFFICIAL standards for only 4 +P cartridges (.38 Special, 9mm parabellum, .45 ACP, and ALL .38 Super ammo) Since there is no official standard is just what IS a +P .380, Ruger is NOT going to warrant a gun for its use.
2. With some extremely light firearms, recoil will be VERY pronounced, When you rachet up the energy, it gets sharper. Recoil can afect more than your hand. There have been some concerns (whether justified or not, I have NO idea) that very sharp recoil in a very light pistol COULD result in the bullets of unfired cartridges moving in the case. In or out would be a bad thing.
Having been told by the maker not to shoot them, I would not. However, those of us that fool with explosives tend to read and follow instructions.