Many thanks to all contributors. The discussion got so technical, I had a bit of trouble to follow.
Basically, I do not have to change anything in a color managed workflow with an adequately calibrated monitor.
I've just received a reply from Saal Digital UK (German print lab with subsidiaries across Europe):
"For the monitor calibration it can leave the L* gamma setting, which is not much different to the 2.2 gamma.If you could sends us pictures or screenshots with the embedded sRGB or Adobe RGB profile that would be perfect."
I also found a page where they clearly indicate that they don't support ECI color spaces (L* based):http://www.saal-digital.co.uk/support/article/srgb-adobe-rgb-1998-and-prophoto-rgb/#sb
They also indicate that they also support ProPhoto RGB (1.
but unfortunately only in 8-bit tiff files.
I think I will stick to Adobe RGB in 8-bit tiff files for the moment.
In a later post, I will share my experience of using Saal Digital for fine art printing.
If anybody knows a good lab that accepts 16-bit tiff files in either ProStar RGB (ECI) or ProPhoto RGB, please let me know.
Best to all.