I don’t think it is necessary to take the Russian threats serious until they start shuffling around the parts of their S-400 system in Syria. That would indicate that they are preparing for something serious. I presume that they are not doing it already.
The Russians can already use their long bandwidth radars to guide missiles. They are using the engagements radars to transmit updates to the missiles.
You might want to read http://www.ausairpower.net/APA-Rus-Low-Band-Radars.html for information about Russian radars and what they might be thinking.