That'd be the same as saying that Luffy won't be a major threat just because they were discussing him at that meeting of shichibukai and other powers. Even though they obscured his power from the public, they still made known that he's nothing to laugh at (which Bellamy found the hard way), where as Robin's high bounty was also meant so that even she told someone the truth, they wouldn't believe her as she was a criminal.
What makes Robin's bounty more significant that Luffy's is that her bounty was given when she was but a child, while Luffy was (arguably) an adult at the time.
It seems to me that declaring a new "threat" is a bad idea for the WG; it makes them look bad. As such, announcing to the world that Dragon or Luffy were "major threats" would cause great concern over the WG's capacity to keep people safe. The information about them would be kept at a minimum and never outside key concentric circles.
The Enies Lobby assault, though, will change that, considerably.
Not so sure about that. They seemed pretty concerned when they were talking about Croc and how he lost to Luffy. I mean, yes, the generic three million belly pirate in the east blue is one that pillages, and going up from there, it's still mostly pillaging villages, so naturally, a safety measure must be taken in the form of the Navy, where as the CP9, the Government's top spy-assasination group, deals with people who pose a much bigger threat to the government, such as those who hold pluton's blueprints (also to gain possession of such documents), or those revolutionaries that Jabura, Fukurou, and Kumadori were sent to kill.
Overt and covert threats. Revolutionaries are covert until they are widely heard; as such, one sends a covert unit to dispose of them. Pirates are loud and make a giant hullaballoo wherever they go; as such, public bounties and official Naval action brings confidence of the people.
Remember that revolutionaries are not necessarily breaking laws; sending in covert agents to kill them sounds like a stifling of natural rights. That cannot look good on the WG's resume.