Hey Maurizio. Not sure if my response ever made it to you, but here goes.
I shoot in studio situations all the time and I often shoot in low light situations with both of the GM lenses you mentioned. I have never had any issues whatsoever focusing. I simply turn off the Live View Display setting, and can shoot at any aperture without any troubles. I've shot wide open with HSS, higher apertures, etc and it doesn't affect my hit rate at all. Your AF assist light should be helping you out to focus when you're in super low light situations anyway. Hope you've figured out a way to make the system work for you.