If there is a limit, it doesn't really matter because messages can easily be overwritten by other HUD messages. You can't do anything to manage this (realistically) since it's managed inside the game itself (and it's probably client-side).
Sending a message occasionally is unlikely to cause any issues and is likely irrelevant with regard to plugin efficiency (assuming you send it at a reasonable rate). Also, I highly doubt that they are the cause of overflows in most cases.