The 787 carbon fiber fuselage allows higher cabin pressure not so much because of increased strength but because of increased stiffness.
An aluminum fuselage stretches like a balloon when it gets inflated, and then comes back to close to the original when the pressure is released. Multiple inflation and deflation cycles cause fatigue cracks, and the higher the pressure, the faster the cracks would form.
Even if the 787 fuselage were no stronger than the aluminum fuselage, its resistance to stretching would allow a higher pressure inside.
Joe