Circa 2013 the USGA introduced a new Stimpmeter. It has two sides. One is the same as the original. The other has the notch at about 1/3 of the length. It is a x2 meter for sloped/faster greens so only half the distance is needed to get the reading ie a reading of 6' = a 12' stimp reading.
The USGA recommend less than 8" between the balls in one direction and less than 18" difference between the two directions.
http://archive.lib.msu.edu/tic/holen/article/2000aug24.pdfI've played with them to see about getting accurate reading on slopes, and as Steve has stated, within reason if the distance is more than 18" between the rolls you can still get a fairly accurate reading. I'm not saying if it rolls 25' in one direction and 5 in other it works, but if they end up 5-6ft apart it will get you within a half foot or so of the exact reading.
Generic stimpmeters give a different reading than the official USGA ones.
As for a green stimping 16ft. I've seen one for a Tour event stimp slightly over 16ft. The players loved it, the Tour didn't...... The greens were not overly undulating and it wasn't easy find an area to get the reading, but it was a genuine stimp of 16ft.
From experience, for regular play speed has an upper limit where it no longer becomes fun as people start to struggle.
The stimpmeter was designed to compare greens on a course, not between courses as there are so many variables that contribute to green speed. As we all know a 10 on an odg course will feel a lot faster than on a large 80s/90s "tournamant" type course.