I think you would find interesting the findings of A&AEE at Boscombe down a fully armed Spitfire Mk IX, ie. the fighter variant tested in dive trials.
As you probably know the often referenced Mach 0,89 dives with were performed with an unarmed photo-recce Mk XI variant. As I recall however this report does not describe the behaviour of the aircraft during the dive, only records the speeds measured with a pitot (which as you noted was not a particularly accurate device at those speeds).
This variant had no cannon stubs or gun ports in the wings, neither it did have the armored windscreen of the fighter variants, ie. it was considerably different aerdynamically than the normal fighter variants. Personally it strikes me as illogical to compare a fully armed fighter such as the P-51 with the additional burden of armament with an unarmed photo recce.
Below you will find the details of the dive with the said Spitfire Mk IX fighter variant. It strikes me that the general behaviour was not at all different from the results with the Mustang in the test you quoted - normal in operation up to Mach 0.70, clear danger signs at around Mach .75, and begun to loose control at anywhere above Mach 0.80+.
If the Spitfire manual said Mach .85 as a limit, it was certainly a very bold suggestion for the pilots.