After doing some digging, here is the scoop per Shimano's Euro website (which I find to be far superior to the US one) The 13 tooth pulleys are to shorten the cage length on the GS capacity derailleur "for better looks as a road component forum." The larger pulleys and slightly longer cage allow for a total capacity of 38T instead of 29T for the standard SGS length. Looks like the 13T pulleys are not retrofittable but are also not a required element for non-triple users.

Cheers
G