After many many hours of investigation i finally managed to undervolt a Z9 mini.
Power usage can be as low as 13.5Watt/Ksol at the wall(power meter) with a platinum power supply and this was a z9 mini batch 2, so all other batches should perform slightly better.
I get 10Ksol with only 138W, which is almost 50% less power comparing to official specs.
Undervolted Z9 mini, can be made silent, with 2 x under 20db fans and a fan simulator
Same performance/watt can be reached on Z9

More details of my tests below

Stock performance

Frequency	Watt	Ksol	W/Ksol
700	363	15.3	23.7
600	311	13.1	23.7
500	259	11.5	22.5
400	210	9.3	22.6

Undervolted

Frequency	Watt	Ksol	W/Ksol
700	321	15.2	21.1
600	237	13.0	18.2
500	173	11.5	15.0
400	126	9.2	13.7

I'm not sharing any how to details because my solution, at this stage, requires additional tools and is too difficult
The software solution will be much better if it can reach the efficiency above.

I've tried playing with the Antminer L3+ solution
https://bitcointalk.org/index.php?topic=3546316.0

but there is no "/dev/i2c-0" file on Z9 mini

Anyone with more knowledge, please share your thoughts on how to get access to those chips through i2c in a similar way it was done for L3+