Might it not be a better approach to identfiy this traffic and figure out the why? Unknown unicast means someone is sending traffic to a MAC that is not known. Is this unknown MAC a real device? This needs to be LOTS of traffic to one MAC, or more probably traffic to many different Unknown MACs. I would suggest the behavior is not beneficial to your network.
I would suggest you indentify the traffic and then as a next step create a FF to block it. The Storm-Control default values will probably work much better.
My 2 cents worth.