Has anyone modeled what the multicast to multiple-unicast efficiency
threshold is?  The point where you go from it being more efficient to send
multicast traffic to individual STAs instead of sending a monstrous (in
time) multicast-rate packet?

2, 5, 10 STAs?

The per-STA-queue work should make that relatively easy, by allowing the
packet to be dumped into each STA's queue...

-Aaron