The FMC is expected to receive a lot more data than to push anything to the sensors. It would only do so when tasks such as deployments, sensor OS upgrades, threat data updates or data restores are in place. However a few tasks can take quite a bit of resources if done too often. Think of the integrations to AD/LDAP realms, pxGRID, Cloud services or Orchestration events. Have them too aggressive, you might need some extra bandwidth to accommodate the frequency.
With that said, and back to the original question, the easy way to rate limit would be how much and how often you push data to your sensors. No other internal ways to limit - and IMHO, on this scenario, applying QoS might make matters worse.
