HI M,
On a Windows machine, you can go into the BIOS and change RAM allocation.
I have found nothing to suggest the same is true on a Mac. It appears it is controlled in ROM, and unless Apple makes a firemware upgrade, it is what it its. Which begs the question, what is it?
Given that the article you cite states the VRAM allocation goes from 256MB to 512MB by increasing RAM from 4GB to 8GB in one example, it would seem that amount might go up to 1GB VRAM with 16GB of RAM. As you're probably aware, Apple bases everything on RAM available at time of production of the Mac, and never goes back to change specs as RAM density increases, so it may also be "locked in" at 512MB for 8GB.
Bottome line: It's possible the base VRAM available may indeed go up as needed on your Mac, but that's far from certain. There is information out there on the internet suggesting this "as needed" theory, so you may want to do further searching reading and see if you can find that needle in a haystack.