I don’t know if we got a Spitfire F.Mk IX in AH, but our Spit IX is inferior to our 109F4.
The very first Spit F IX model was designed as a high altitude fighter. That's what we have in AH.
The F4 has a critical altitude of just under 20,000ft. The early Spit F IX 27,400 ft
If you compare the charts for the 109G2, which was the early Spit IX’s contemporary, you will find that the 109G2 is clearly superior in every respect except turnrate and top speed above 25k.
The Spit F IX had a climb rate advantage over the G2 at medium alt and above, and rough speed parity at lower alts. Above 20,000ft it is faster, better climbing, and of course retains it's turn advantages.
The problem is AH models the 109G2 at 1.42ata, when for most of it's life it was restricted to 1.3ata (see Neil Sterling's post earlier)
If you compare the Spit F IX to the 109G2 in the Finnish tests, at 1.3ata, the Spit is clearly superior.
As you can see the 109G10 is only somewhat faster up to 12k where the Spit XIV’s speed suddenly drop off and the 109G10 gains a significant speed advantage. The Spit XIV doesn’t regain parity until 25k, between 25k and 27k the Spit XIV is faster, but above 27k the Spit XIV again loose the advantage to the 109G10 as the Spit’s speed drops off sharply and its WEP is ineffective.
AH models the G10 with C3 fuel and MW50. In real life, the G10 usually flew with B4 fuel and MW50, with a power output of 1800ps instead of the 2000ps those figures refer to.
AH models the Spitfire XIV running on 100 octane fuel, at 18lbs boost, with arouind 2000hp. In real life, by late 44/early 45 they were running on 150 octane fuel, either 21 or 25 lbs boost, with a maximum power of between 2300 and 2500hp.
I claim this to be parity given the better turnrate of the Spit XIV.
In real life, you could safely knock off around 200ft/min from those G10 figures at low altitude, and even more at altitude.
The 109K4 climb chart that Pyro posted here a year or so ago shows very different figures, especially at altitude.
For example, at 30,000ft the 109K4 climbed at around 1900 ft/min, way below the Spitfire with 2350 ft/min.
At 25,000ft Pyro's chart puts the K4 on 2,660 ft/min. The Spitfire test reports put the Spitfire at 3,150.
At 20,000ft the K4 did 3385 ft/min, the Spit XIV 3650 ft/min.
AH seems to have given the Spit XIV too high a rate of climb at low altitude, and too low at high altitude.
However, on 150 octane fuel, the AH figures are probably too low for low alt, and still too low for high alt.