Just to report that i did update successfully my M20S with the latest firmware and the machine has been running well for more then 48 hours.

From the support page, it says "The latest firmware optimizes machine power consumption and enhances machine stability". Convinced me to give it a shot. It was the first time i was updating a firmware on a WhatsMiner, so i'm going to give some details if that can help someone.

On this page i could see that a new firmware was release on December 11th 2019:
https://www.microbt.com/server/firmware.html

Question was do i own an H3 or H6? Looking at the miner externally, does't seems to be any reference to the control board version. Thought i was going to have to open the casing but found an easier way. First, i installed the WhatsMinerTool-4.2.5 (from support page) on a Windows machine. Then i scanned the ip range and found the miner. Finally looked at the listed M20S miner (scrolling right in the list) a column labeled "Version Info" that shows a long code beginning with H3 or H6.    

Knowing i own an H3 M20S, downloaded the upgrade-whatsminer-h3-20191211.17.bin file from the firmware support page. Used the "Upgrade" button in the Windows tool, selected the .bin file and completed the upgrade process with success.

GH before firmware update (original firmware) : 67500
Power before : 3268W
GH after firmware update : 68700
Power after : 3221W

So, i roughly gained 1 TH for the same consumption.