Many times a load does chronograph slightly faster in fired brass than in new brass, but it has nothing to do with higher pressure. Instead a little bit more of the energy from the powder is used in "blowing out" the new brass to fit the chamber, rather than pushing the bullet. This effect is slightly larger in fire-forming improved cartridge with new brass.
Fired brass might result in slightly higher peak pressures due to working-hardening the neck, whether through firing or resizing, resulting in the neck not releasing the bullet as easily. We know that "cold welding" of a bullet in the neck can result in higher pressures.
Or at least those are the conclusions of some shooters, including at least one pressure-lab tech I know.
