Nope, or at least none that called themselves such. We tried neo4j, which exploded trying to import data on the order that we're working with and a couple of RDF databases, which survived the import, but were a couple of orders of magnitude off from the performance we were hoping for.

After writing some 8 different backends for our store class and none being within an order of magnitude of our own prototype for the sorts of applications we're doing, it seemed more fruitful to round out our own application rather than continuing the seemingly endless recurse of possible data backends which ranged from mildly to amazingly disappointing.

If you've got something specific that you've worked with in the past that you think would be worth our while to evaluate, I'd consider investing the time to try it out. But just that there exist more options that we could evaluate at the moment doesn't necessarily imply that it's reasonable to keep writing new backends, which sometimes take a non-trivial amount of effort.