Further to the above, I had provided my findings to Canaan and received this email in response.

e.g. there is an option to set a -2 to 1 voltage offset, this worked with the 741s, but with the 821, that option is there in the web interface, but the command that is actually used to start the miner,
*always* forces the 0 offset and ignores the value set in the web UI.
>> This option is for A741, sorry for we didn't delete this function from A821's firmware.

There is one other around the min fan speed, it defaults to 100%, what probably makes more sense is to set to that 50 by default.  The outlet temperature is set to 70, where as on the 741 it was 90, because of this with the default min fan set to 100%, the fans run at 100%  speed whether they need to or not.
>> We locked the fan speed in the A821 firmware since its chips cannot take high temperature.


Again, I wouldn't change that 70 on the temp... the fans shouldn't be a problem and it appears the undervolt will be coming out soon