After posting this I was a bit troubled by the statement that a Spitfire
Mk.XII
attempted a split-S from 1,000 ft. AGL.

Good notice of this error. Minimum altitude in Mustang would be 2000' to give
some safe margins for recovery. Its laminar wing is not as capable of minimum
radii "half-loops" as the Spitfire's wing. I'm not sure if this correlates with
any sim modeling.
VL
PS: Throttle to idle does not get the minimum radius. Maximum G limit AND
minimum TAS provide minimum radius "loops". The maximum G is the structural
limit and the minimum speed at which the maximum G could be obtained is the
target TAS. This target TAS sometimes requires the addition or subtraction of
power to obtain and or sustain. In the Mustang it is 270 mph.
Hope this helps.