Canaan has released new OpenWrt and MM firmware for the A821, A841, and A851. They claim in an email (the same email that announced their flash sale) that the new firmware improves efficiency to up to 85 W per TH/s.

OpenWrt firmware

AvalonMiner 821: https://canaan.io/downloads/software/avalon821/openwrt/20181128/
AvalonMiner 841: https://canaan.io/downloads/software/avalon841/openwrt/20181128/
AvalonMiner 851: https://canaan.io/downloads/software/avalon851/openwrt/20181126/

MM firmware

AvalonMiner 821: https://canaan.io/downloads/software/avalon821/mm/2018-11-26/
AvalonMiner 841: https://canaan.io/downloads/software/avalon841/mm/2018-11-26/
AvalonMiner 851: https://canaan.io/downloads/software/avalon851/mm/2018-11-26/

There is no changelog for the OpenWrt firmware at the time of writing.

The changelog for the MM firmware simply states "support for low power."