Even before ECA, the latest (MProse) patch should preclude operation of flaps at 'combat speeds' (or perhaps 'combat proximity'?) by all but the named planes VGun gave you.
In any case, the only time I ever heard a WWII vet talk about using flaps was an old P-51 jock (Kitringen or Kitriger or something like that) who had really got his fangs out in a dogfight with an Me-109 which he had chased all the way down to low altitude.
Seems they ended up in a knife circle going 'round and 'round when he 'suddenly remembered' and used the flaps to start gaining for the kill. He was _stupid_ in entering such a prolonged battle and admitted as much when he said his first-worry was an external bounce.
Not to be arrogant but use of flaps is a /lift/ bias gambit that has ZERO relevance to the high speed fight that is the _only_ 'sweet zone' you want to be flying in against multiple human opponents and especially with a human wingman to help you.
High Speed you fight-by-G, whatever the height, and though the propjobs didn't typically have the either the power or the instrument indication to give you a 'straight' or for-AOA G clue; you can generally tell within a second or two (by relative crossing rates) whether a guy is gonna beat you for lead and so if you should try for a snapshot or lag out and possiby use a /small/ out of lane move to stay inside the radii on a high crossing secondary opportunity.
If, at the end of this second attempt, you can't latch to at least to his 3-9, you should unload and extend whilst having your wingman take advantage of the defensive Lufberry wheel/break you've likely forced the badguy into.
Again, props simply don't have the umph (for drag) to 'play up' from an endless, horizontal, circle like jets do.
Certainly, in the instance you mention, the 109 will be VERY nose heavy coming back down and it's pilot will be fighting that with a small stick throw trying to remain 'unpredictable' (defensive) or 'tracking aggressive' (nose-on-you) with his Immelman exit to the half loop.
This means a shallower spiralling climb on YOUR part will keep your deflection constantly changing on his own nose point selection with a higher-sustained energy value for chasing when finally brings that damn DB-605 /brick/ back below the horizon.
I should also mention that the Spitfire is -unmatched- for the spiral-climbing turn.
If he's /not/ at zero energy himself and you chase him until you are, YOU are _screwed_.
Becase in real life, going vertical to zero airspeed and /then/ popping flaps is asking for a pitch authority change that can 'solidify' the elevator effectiveness on nose point (deflection for that snap guns lead equation) at the same time acting like speed brakes even as the effective airfoil AOA is changing.
Such that you may very well depart completely as the last bit lift leaves the wing. Add a little roll and you're spun as well.
Do this 'very often' (once) and the guy up-top will simply drop onto your tail and gun your brains out while you twirl downwards.
It's gonna be a long war. There is absolutely no such thing a 1v.1 'chivalry' but there /will be/ plenty of targets...
SURVIVE (and _use_ your wingman to tag team the other guy's energy account).