This won’t be a large post at all; rather a quick follow up to a my first post on this topic. To summarize: If I have 1 4K/144Hz and 2 4K/60Hz displays connected to my NVidia Titan X Pascal card, each display works at its maximum refresh rate. If I connect a fourth display to the card, the 144Hz panel drops down to 120Hz.
Today, I undid the SLI configuration of my system, so that I had 2 independent Titan X Pascal cards. I connected one of the 4K/60Hz displays to the second card to see what would happen. Sure enough: with that configuration, all 4 display ran at their maximum refresh rate. So the long and short of this is: it does appear to be some sort of limitation in a single card or GPU. Since I’m running an SLI setup normally, I can’t keep the cards independent like this. It was just done for testing.