I'm not sure how intensive the rebuilding of the bitfields is or if I can make it just rebuild the bitfields for my product alone.
Ideally the math should be done once only. Because, 500 users concurrently on your forum might just make the pow function the straw the broke your poor server. So, it should be done once and then stored somewhere for future use. If you wish store it in the database, or store it along with the XML file.
I have no idea about the rebuild bit fields myself, I'm not a plugin author for vB. But should I stumble across the answer, I'll be sure to post it back here. Until then, I'll leave you in the capable hands of the community.