Did oil firing raise top speed or sustain it?

Did changing Queen Elizabeth from coal to oil chiefly increase maximum speed?

Its clearer benefit was sustaining full power. Larger gains required small-tube or later high-pressure boilers designed to exploit the hotter fuel.

Fuel type and boiler design have to be considered together. Oil can burn more cleanly and consistently than hand-fired coal, improving the crew's ability to maintain high output. The historical ships still used large-tube boilers rather than the small-tube and high-pressure plants that would later extract much more power per ton. The conversion chiefly improved sustained output, while much larger speed gains required a different boiler plant.