Hi,
Thanks for your advice. Indeed I initially thought about this Profile 1 stuff and forgot about it in the meantime. But this helped me re-think about how I have installed my RAM. And the solution came out of this ! They wasn't installed in the proper dimm slots to make available dual channel. I felt like a complete idiot because it took me so much time to realise my mistake...
Now I have those results :
View attachment 37018
And by the way, changing to MacPro 3,1 or iMac 12,2 didn't make any difference, even SSDT stuff.
Now I can install my other HDD and get a mounting for my 2,5 SSD and write a post about my build