Hello camber,
Here is the description of unmodified variable and fixed datum boost control for Merlin II and III engines:
http://forum.1cpublishing.eu/showpos...62&postcount=8
http://forum.1cpublishing.eu/showpos...66&postcount=9
http://forum.1cpublishing.eu/showpos...9&postcount=10
In Short: All Merlin III and majority of Merlin II had variable datum type boost control. It's save to assume that a typical Spitfire I had a variable datum type boost control (except of a really early one with fixed propeller).
The cut-out disables the boost control and throttle valve is controlled directly, however the modification contains a hole that prevents the boost to raise above +12. In case of a malfunction of the boost control the cut-out could still be used for it's original purpose even if modified.
The Spitfire II had both, a take-off gate and a boost control cut-out. Take-off gate was used for take-off boost only, the boost control cut-out for emergency (=combat) boost. "Pilot's Notes General" is very specific at that point, see:
http://forum.1cpublishing.eu/showpos...00&postcount=5
So in combat with the 109 the pilot wouldn't go "through the gate" but use the cut-out.
Note that for later Merlins (my guess would be from 60 series onward) the boost control was not disabled for "emergency boost". The "cut-out" set the boost control to maintain a higher value at each throttle setting.