The projected sixth Queen Elizabeth as an early fast battleship

Could the projected sixth Queen Elizabeth-class ship have been the first fast battleship?

Yes, if small-tube boilers had enabled about 28 knots while preserving the class's battleship characteristics.

The unbuilt follow-on could have become a genuine fast battleship. Small-tube boilers might have supported roughly 28 knots, with the relevant machinery later appearing in Renown and Repulse. The rest of the design would retain battleship characteristics. A propulsion change could therefore cross the speed threshold without adopting battlecruiser internal arrangements.