I'm not sure where you get the idea that R-26 is not temp stable. Inferior R-23 according to the Alliant reloading is often as much as 100 fps slower in any given caliber. In 243 as much as 170 fps down. I don't really have a reason to drop down to the faster burn rate R-23
I was using R-25 for years when R-26 came out. Reloading manuals are also just a ruff guide and not the truth. They are often biased to the powders that paid the most. So it is not wise to quote reloading manuals. Chrono's don't lie. I work up everything with chrono data and often download for barrel life, accuracy, and consistency. I also a longer barrel person to get extra FPS and take it easier on the cases.