I must have gotten lucky. My iPhone 15 Pro has been running incredibly cool since I transferred 300 GB of data to it, even during a long phone call with a weak signal, where my iPhone 14 Pro would have warmed up a lot.
I suspect many of these reports may simply be related to people restoring their data and setting up their iPhone for the first time. With a large photos library, that process can take a few days to fully complete, index all files, and clean up after itself.
Right now, I’m thinking this ends up being a total non-issue. Am interested to see in-depth testing though.