I'm the maintainer of the Google+ Hacker News Circle (Well, the script/cron job that makes a circle from hngp.axxim.net). I increased the frequency to every five minutes to keep up with the new growth.
It's great that you're finding a use for it, but it's still irritating that the implementation of shared circles doesn't work well since you can only share a snapshot that users have to manually re-add in order to update.