Sepherosa Ziehau has posted a detailed message showing the speeds he gets with multiple transmission queues, using igb(4).  The short version:

Quick summary, the multiple TX queue support gives me:
+200Kpps for 2 bidirectional normal IP forwarding (now 4.40Mpps)
+160Kpps for 2 bidirectional fast IP forwarding (now 5.23Mpps)