What fuel penalty bought four more knots?

Would the extra machinery needed to raise an Italian heavy cruiser from 32 to 36 knots roughly double its fuel consumption?

Probably, though a larger Mediterranean design could devote more internal volume to fuel.

The proposed increase was from 32 to 36 knots, not a ten-knot leap. A third turbine would raise the plant to roughly 137,500 shaft horsepower. Moving from an eight-boiler, two-turbine arrangement toward twelve boilers and three turbines could roughly double fuel use. A purpose-built larger hull could carry more fuel, while Mediterranean operating distances made the penalty easier to accept than it would be for an oceanic cruiser.