I do this now quite a while now and I notice that the devs have practically a impossible task.
I notice this with every aircraft I compare against RL flight test.
Since the comment of Mr JG something something got me curious I tried to find some speedtests I could find with that Power rating. I found exactly two matching 11lb and another one for 12lb with 150 octane fuel.
I initially agreed that the 11lb tempest seems to fast at alt.
The first I found was a 11 Lb test from March 44. and that one is indeed some what slower than my ingame tests.
I found another one from an unknown date which is MUCH faster down low. Over 640 kph on the deck. But has no records up high.
Another one at 12lb is basically matching ur Tempest down low. But has no records up high again.
All that tells me that the modelling of top speed is a lot of guesswork and reading the reports of the tests carefully since you never know how the tests were precisely conducted.
Were the surfaces treated? Were there some bombs racks attached? Another propeller? What was the weight of the plane? Was ammo in the weapons?
Where the weapons even on the plane? You get so many variables, that you cant compare most planes 1:1 against the game version.
And even knowing that, you cant be sure that the engineers back then made no mistakes correcting their results to standard atmosphere correctly, and for what they corrected it.
Did they removed in their calculations some mirrors on the planes? some weapons?
I test my planes most of the time with half fuel, to simulate the most realistic encounter, but I vary that for planes with a lot of fuel. (P-51, P-38 and P-47 for example.)
So another variable.