Since I'm retired, I don't have access to myboeingfleet.boeing.com so I don't have any idea how CFMI supports their position. But in any case it's at odds with the relationship between thermodynamics and metallurgy that I was immersed in for a few decades.
The rule of thumb I recall is that a 10 C drop in EGT doubled HPT parts life, and this was exponential; 20 C was worth 4x parts life.
EDIT: Thanks,
Mutt, for the clarification.