Well the thing is this has already been done.
Lots and lots of stuff here in this thread:
http://forum.1cpublishing.eu/showthread.php?t=26956
Hi. What is relatively accurately be measured, is the speed at sea level. (mph) (RL datas from Spitperformance):
Hurri D-5-20. In the game 240 instead of 262. -8,4%
Hurri Rotol. In the game 260 instead of 265. -1,8%
Spit I. In the game 240 instead of 283. -15,9%
Spit Ia. In the game 240 instead of 283*. -15,9%
Spit IIa. In the game 300 instead of 290. +3,4%
Fiat G.50. In the game 223 instead of 248. -9,9%
Messer E-1. In the game 273 instead of 302**. -9,4%
Messer E-3, E-4. In the game 273 instead of 290***. -5,7%
* If I am wrong in this, then I apologize. I do not know well the Spit subtypes.
** (edit) The measurement of this ratio does not matter, but it's good to know: this is not the 109's top speed, is only 1.35 ata boost pressure, instead of 1.45 (this is called the "start und notleistung"). That would be 200 PS power (~ 20%), which increases the speed only 10 kph (~ 2%) in the game.
*** Performance tests in RL are possible margin of error of ±5%. Maybe this is why measure at slower than the E-1, despite the fact that the E-3 is more powerful engines were built. Or the E-1 graph is bad. Who knows?)
Although I believe the in game max speed for the 109's is 460 kph/286 mph. Still too slow though!