Frankly I have the 60mm macro lens and used it behind the flat port... afterward I used it behind the 4" Nauticam port and found that I get 30% better FOV and get the same magnification by getting closer to the subject.
As I have the flip macro adapter for this port (I use it especially when I use the 12-50mm lens) I get supermacro details by using it with the 60mm macro lens.
No quality issues that I can tell.
BTW the 9-18mm minimum focus distance is 20mm so it is clearly no problem with the focusing on the 60mm lens.
It is bigger than the flat port ... but that is the only negative i have to deal with vs. the flat port...