The Spitfire V is currently using a 16lbs boost model which indicates it's a Merlin 45 A.A.878 variant engine. This engine is rated for combat mode for three minutes only before maximum engine temperatures are reached. The current model does not reflect this condition.
At climb speed, boost is at 16lbs up to 8800ft where it drops continually.
At level speed, 16lbs of boost is available up to 13000ft where it drops continually above this altitude. See charts below for more info.
Boost and Airspeed Tables:
Fig 1. Performance on ClimbFig 2. Level Speeds and Boost at HeightsResources and references:
RAF Aeroplane and Armament Experiment Establishment from July to September 1942. Rubbra, A.A., Rolls-Royce Piston Aero Engines – A Designer Remembers, Rolls-Royce Heritage Trust, 1990
Bingham, Victor, Major Piston Aero Engines of World War II, Airlife, 1998
