Is the maximum power the Merlin 61 reciprocating engine occuring at its maximum RPM?
Power output of a reciprocating engine goes up with speed, and yes, maximum power is reached at maximum RPM. Considering the size and age of the Merlin 61 (27 liters displacement) it had a rather high maximum speed. Some contemporary engines ran even slower:


Daimler-Benz 601E: 2700 RPM for take-off, 2326 RPM continuous power
Daimler-Benz 605A: 2800 RPM for take-off, 2300 RPM continuous power
Allison V-1710-A: 2800 RPM
Klimov VK-107: 3200 RPM for take-off, 2800 RPM continuous power
Jumo 213A: 3250 RPM for take-off, 2900 RPM continuous power


Note that the considerably smaller GA engines still only allow a maximum speed of 2700 RPM. The smaller the engine becomes, the higher the maximum RPM will be. Model engines for RC racing easily top 50.000 RPM.

Age is also a factor. Modern engines run much faster, and the early rotary engines of WW I would run 1350 RPM, max. The radial engine of the Boeing Stearman which I once flew was limited at 1700 RPM, requiring me to throttle back when accelerating for a loop-the-loop so I would not overspeed the engine in the dive.