and the range with this mod was equivalent to the Mustang.
There is an error in your assumptions of Spitfire range vs. Mustang range and it is based on the data you provided. The internal fuel of the Mustang III in the report is 150 gallons, (UK). This is the amount of internal fuel, all wing tanks, of the P-51A and some P-51Bs. It does not include the 85 gallons (U.S) fuselage tank in the majority of P-51B and Ds. Apparently, the British Mustang IIIs did not provide the fuselage tank, the one the data page references certainly did not. An accurate comparison, converted to U.S gallons and the requirement to burn off some fuel from the fuselage tanks (est. 20 gallons), of the P-51B to the Spitfire data page would be;
internal fuel P-51, 249 gallons,
Spitfire (data page), 144 gallons, and
Spitfire (drawing schematic) 194 gallons. It is apparent that the P-51B/D carried substantially more internal fuel than either version of the Spitfire, or, 105 gallons more for the first and 55 gallons more for the latter (for a comparison, this is half the fuel load of a Bf-109).
Internal fuel is critical for an escort fighter. When combat is initiated all external fuel is normally jettisoned. That means that the fighter now must fight with internal fuel but not use the fuel required to go home. I estimated in a previous thread that the P-51 would get about .6 minute of combat for each gallon used. This would equate to an additional hour of combat for the P-51 over the Spitfire version on the data sheet and an additional 30 minutes of combat time for the P-51 over the Spitfire version in the tank drawing. In addition, according to the data provided, the P-51 is 11% more fuel efficient so it would not need the same amount of fuel for the home trip, thus increasing its time on target.
It is apparent that, while the two versions of the Spitfire would be capable of the escort mission (the first marginal), their time on target would be significantly less. Also, since the Spitfire in the drawing has aft tanks like the P-51 fuselage tanks, some cg fuel burn off, like the Mustang, might apply, again affecting time on target.