Plugging in a Save-kun (which is the same thing as MB128; it was named differently just because it was sold by Koei) won't give you typical backup RAM function though. You could only save with games supporting(or requiring) the MB128, so if the PAC didn't have built-in backup RAM (which it did have) you're screwed up, unable to save in most of the games(and a large portion of the PCE library are RPGs).
As for SG support, IMO they should instead include SG support in the original DUO already, as it was meant to be the ultimate all-in-one system (and signified the death of the core concept as they eventually decided that you didn't need the expansion slot to connect to any other things). They just gave up the SG completely really early, long before even someone would consider making a CD game with SG support.