I can’t answer your question about heating, but I am curious if you purchased your kit recently and if the display is working for you? I bought two kits recently from Adafruit, they wouldn’t work, Pimoroni sent me two replacements, and I’m having trouble with those too. It seems like there is an issue internal to the displays themselves, nothing wrong with the Pimoroni controller.
Don’t mean to hijack your thread, just curious if you’ve had success with your display :)
For what it’s worth, during my troubleshooting on my displays, the screens (on the rare occasion I’ve had a workable image) barely warmed up at all, the controller got a little warm but nothing crazy. For my project I plan to have the controller exposed to the air just to be in the safe side, but certainly no forced air cooling. I would guess that at least some means of heat to get away from the controller would be wise in your case, but it probably doesn’t need to be totally exposed as mine will be (assuming I can get the displays to work that is…). These are just my guesses of course, hopefully someone with a bit more luck with these displays can give you a more experienced-based answer.