That is definitely true of the later Mamod TEs but not true on the early 60s TE. My experiment involved a non-adjustable TE frame mounted with a 1980s Mamod "locomotive" double acting oscillator cylinder. The double cylinder clearly had double the power but it also clearly consumed twice the steam. Admittedly my sample size is small.The simple Mamod traction engine has variable cut off, using the eccentric 'throttle' changes the position of the cylinder oscillating axis, which in turn changes the period of the inlet.
Maximum efficiency of a boiler is achieved using maximum superheating of the steam. The energy required to boil water (latent heat of evaporation) is required whatever the boiler pressure. Additional heat used to increase steam temperature and pressure is realised during the expansion phase and is delivered as output power.
The most efficient steam engines are supercritical, where the steam is superheated beyond the point at which the pressurised stem is as dense as water, allowing for efficient heat transfer. This happens at pressures on excess of 3000psi and temperatures in excess of 400 degrees Celsius. These figures are only realistic in flash steam boilers, where the pressure vessel consists of small diameter tubes which have high strength relative to their inside surface area. An interesting topic, which I intend to pursue at some point.