Well, I've found the sweet spot of the GPU thanks to the tips of people here to have a better cooling,
and my personal tests, and in the end I've managed to have a "silent" 1080p60 system.
I am so happy with that. In fact my main gripe with the PC was the noise of the gpu fans at high speed when I didn't have internal cooling in the box.
To achieve that I kept the default clocks -maybe I could increase the clocks a little because the power draw is never going to be more than 102W, reduced the power draw by a 14% and set the max voltage to the minimum. The results are great. The GPU never goes over 102W of power consumption and all games run at 1080p60 as before.
What's more surprising is that the benchmarks give the same results overall.
Since the power consumption is low and 71-72º is the max I got with The Witcher 3, my most demanding game, the gpu fans run silent. In fact, the only continuous sound now is the internal cooling, which is kinda soothing and chill -it also hides the sound of the 3,5" HD, which can be annoying- :smile2:. Whether I play my non demanding games or I power-game, the sound is the same overall, and that for me is quite the achievement, because I don't like noisy devices.
These are the settings. They are fine tweaked, maybe the clocks could be slightly increased but that's it, because a 1% less power limit and most games will run but at -15% it is unstable with Sky Diver test -not with Fire Extreme for instance-:
Idle temps went down from 47-51ºC to 31-29ºC.
As
@Clukos would say, undervolting is the actual overclocking. SO HAPPY.