Apologies if someone else has already asked this and it’s been answered, or if it’s just a daft question, but are there any thermal/electrical/height reasons not to stick a pimoroni supplied 6mm heatsink on the new Pi-3B+'s? (My poor old eyesight can’t tell if the little circle on the top-left of the SoC is a vent hole to allow gas expansion inside the heat spreader, or just a bit of paint for use as an orientation mark).
Poking a small object into the tiny tiny circle confirms it is indeed a hole, and gas expansion would be a logical reason for it to be there. I’d be wary of sticking a heatsink over it, I’ll see if I can find any details.