From the captures I've done, all of the control frames are being sent at 24 Mbps: ACK, Block ACK, RTS, CTS, etc.
Yes, bc/mc optimization is configured. This must be what's doing it as you stated.
The reason I've been looking at control frames is that I'm seeing what seems to be a disproportinate amount of control frames to data and management frames. RTS/CTS is making up about 50% of the frames at times. If considering air time, they're consuming about 1/3 of the air time. So at any given time, the majority of traffic captured is running at 24 Mbps, but that's considering Block ACK, ACK, the stuff that is expected. I'm not sure if I should be alarmed by this or not.