What am I doing wrong? 2-stage prop spitfires.

[Space Communist]

Ok so I have put a number of hours into the game and I have gotten I think pretty proficient at flying the Hurricanes and Spitfires with variable pitch props (CEM on.) I am able to fly a Mark II Spit level at about 300 mph IAS, which more or less agrees with the real top speed as I understand it.

Now the Ia with its 2 stage prop should actually have a pretty similar top speed in the coarse pitch setting I would think, but I struggle to get it to 250 mph, normally no more than 240!

now I know some people have been claiming some of the flight characteristics are a little off but is there really a 60 mph difference between these 2 planes? If not what on Earth am I missing?

Edit: Looking up stats it looks like the 2 aircraft should have nearly identical top speeds, though the speeds I have found are at 17000 ft, and I am testing at about 4000 ft. Can the prop make that big a difference between low and high altitude? i.e., would the difference disappear at higher altitude?