Hi Malte,
there is work going on at "one of the companies using OpenACS as a base system" (not us! 😊 ) to release a brushed-up version of "ACS Attributes" as GPL.
This module integrates with the category module and stores the category values by modifying the actual SQL table (the mechanism foreseen with acs_attributes). This has a considerable performance advantage, compared with the "skinny table" storage of the category package.
Bests,
Frank
