Quote - In most cases a prop is just an obj and a texture. I'm wondering how significant the memory savings would for an average prop to reference an external obj.

Very minimal.

External geometry for props is mostly something people do because it makes development easier (OBJ can be replaced without disturbing the PP2) and was particularly useful when making a set of PP2s for material variations (back in the P4 days when this was the easiest way to change the materials on a standalone, unparented prop).
It's also a (notional) requirement of most Poser brokerages.