RL26 temp sensitivity

[EdP]

That round would need to sit in the chamber for a bit for it's temperature to adjust all the way through.

It doesn't have to reach equilibrium all the way through to have an effect. The majority of the powder is what is closest to the brass, which heats quickly. The bigger point is that the powder mfgs developed a better product to address the issue. It may not be an issue that affects you and your shooting, but if it does then using the improved product makes one less thing to be concerned about.