Whatever bus video runs over is wired through the dedicated GPU, so integrated is not an option with external monitors connected. That by itself would be fine, except for whatever reason, driving monitors with mismatched resolutions maxes the memory clock on the 5300M and 5500M. This causes a persistent 20W power draw from the GPU, which results in a lot of heat and near-constant fan noise, even while idle. As there isn’t a monitor in the world that matches the resolution of the built-in screen, this means the clocks are always maxed (unless you close the laptop and run in clamshell mode).
The 5600M uses HBM2 memory and doesn’t suffer from the issue, but a £600 upgrade to work around the issue is lame, especially when I don’t actually need the GPU, you just can’t buy a 16” without one.
Disabling turbo boost does help a little, but it doesn’t come close to solving it.