According to Mongo's documentation, the following is used to construct an ObjectId:
ObjectId is a 12-byte BSON type, constructed using:
a 4-byte value representing the seconds since the Unix epoch,
a 3-byte machine identifier,
a 2-byte process id, and
a 3-byte counter, starting with a random value.
So to answer your question,
Is there anything else an attacker could gain from knowledge of the
I would say that it seems like in addition to the time stamp they could also determine the machine identifier, the process id and your counter value.
Make sure that you are protecting yourself against direct object reference attacks (which you should be doing anyway).