While near the upper limit of the performance window, Hesp's results seem entirely plausible to me.
My .264WM shows first signs of pressure at 74.0gn of RL33, and that is being conservative, i.e. looking for visible pressure signs with a jewelers loupe. This with a factory barrel, but not a Barnes bullet like Hesp is using, so velocities don't correlate.
Different air temperatures, different powder lot, different barrel (bore surface, twist), different chamber, especially a custom chamber, could all "stack" up to easily digest another grain of powder w/o pressure issue.
Don't under estimate the .264WM. This classic, and always great cartridge is like a new and even better cartridge with modern powders and modern bullets.
JMHO
