I have owned a few vintage Thuringians, and a number of PHIGs... The PHIG that I am down to is finer than any of the thuringians I have ever had....
I have never uses a Thuringian but my C12k is a great finisher, very hard ( a bit slow but no worries as a finisher) with a crispy sharp edge. I prefer the finished edge smoother but it's a great stone. especially for the money. I just use mine with water.