Why speeding up the Revenge class was expensive

Was fitting small-tube boilers to the Revenge class seriously considered in the 1920s to increase speed?

They already had relatively small-tube boilers, and the machinery changes considered were expensive because space was more restricted than in the Queen Elizabeths.

The Revenge class already used relatively small-tube boilers. Various changes to improve performance were considered, but the ships lacked the space available in the Queen Elizabeth class. That restriction made a substantial machinery improvement expensive.