The last test report on primers I saw was quite a while ago- maybe 1980's - the Fed 215 was not actually the "hottest" per say at that time, but had the longest duration of flame-out - which developed the higher temps due to the time duration of the flame - thus was better at igniting the slower powders.
This longer flame front was why it was the preferred primer for the larger cases & slowest, hardest to ignite powders.

Times change and so do components over the years.