You don't need to burn off the lot - just the volume that causes pushes the CG envelope into the restricted area. I don't have the numbers handy but as example on the Mustang the rear tank restrictions only applied when you had more than x gallons in that tank. Taxi, takeoff and climb to above your intended target so you can hit them at full speed instead of climb speed when an interceptor is going to burn a lot of that off and probably move you out of the restrictions though maneuvering will still suffer while you have fuel sloshing around in that tank.
You are quite right but you do have know which airplane you are dealing with and what mods may have been done or not done to the planes.
One some of these planes the take-off and climb to somewhere around 5,000ft was done on the main (normal) tank/s and then the rear fuselage tank was drawn down to the "safe" level before the drop tanks were used. On a P-51 that still left a lot of fuel in the rear tank.
Then you can get into arguing about which radios were fitted where, any other extra gear, different safety margins between war time flying and peace time and other things.
Not having flown any of these planes my opinion doesn't mean much but I think the Spitfire could have used some sort of rear tank. The question is how big a one and when.
Look at the games they played just fitting different propellers to the MK I Spits and the different amounts of ballast that was used, Wooden stick got lead shoved in the engine mounts tubes and other weights. two Pitch props go the lead taken out, DH prop with constant speed pump fitted had different ballast, Rotol prop had even more ballast shoved in the tail. That is if I remember right. Four different weight set ups just for the propellers on the MK I.
Now which Spitfires got the tanks in rear of the fuselage? Any difference in the engines? A Merlin 66 or 79 was supposed to weigh 1645lbs while a Merlin 266 was supposed to weigh 1675lbs, I don't know if the Packard was actually heavier or if they included something RR didn't. Engine weights were not always the same, Allison included things like the exhaust flange, gasket and nuts/washers but the exhaust pipes were the responsibility of the airframe maker for instance.
Also consider that even a MK IX Spit was a 7250lb airplane with full normal load, A P-40E with only 37 gal (US) in the rear tank was over 8000lbs and a P-51B without the rear tank and clean was approaching 9000lbs. Now lets stick a tank with even 25 imp gallon in the rear fuselage of all three planes at the same point aft of the CG and see what happens to each plane.
And we are not considering the tail plane size, the elevator size and any trim tabs to help control things when they get out of whack.
Saying the Mustang could do it so the Spitfire should be able to it too ignores a whole bunch of things. Now maybe you could modify the Spitfire to improve things.
My friends full size pickup truck can tow a 3500lb trailer, the trailer will fit behind my Hyundai Tucson and you could get the hitch to line up. Doesn't mean I should drive the rig on the hi-way let alone try to drive it down hill.
