How could orientation and cabling affect heat??
At some point during all my fiddling with this stuff, I discovered a correlation between GPU power consumption (as indicated by iStat Menus) and fan speed. For example, if I switched to high resolution, the power would increase, and at low resolution, power would decrease. And of course more power means more heat, which causes the fans to run. That kind of made sense: moving more pixels consumes more power.
But I also observed an effect caused by refresh rate. Oddly, I seem to recall 30Hz would cause increased power draw but 60Hz the power draw was reduced. Yes, this seems counterintuitive. Since I didn't have a good model for what caused the increased power draw, I decided to try all combinations of the settings. On a lark I tried inverting the image and it actually affected the power draw! Probably has something to do with the access order of memory, but I don't really know. And indeed it seems odd that running the image upside down actually pulled less power than right side up.
The cabling issue was a bit more complicated. My initial configuration was to run the video through a dock, with the dock's video output going to the monitor. That always resulted in high power draw. Connecting the computer directly to the monitor did have the effect of reducing power draw... but only in certain configurations.
eg. from personal experience, on older Nvidia cards (1080 Ti), just adding +1Hz to the refresh rate (or using a slightly higher resolution, or 10-bit color, or no DSC etc. etc.) caused it to never enter low power mode, changing power consumption permanently from 10W -> 50W.
AMD GPUs on even the most modern Linux kernels don't enter low power states properly. If you do change the power profile to do it more often, browsers become a laggy mess.