I haven't finished the download yet but if BoP is doing things right then the speed you're seeing listed is called Indicated Air Speed (IAS). The top speed of aircraft that you see published in books and so forth is listed in True Air Speed (TAS). The Mustang will top out at 437mph TAS (D model) or 439mph TAS (B/C models) in the 25,000 feet range where the difference between IAS and TAS will be quite high. At sea level IAS and TAS are the same.
The difference is that air at higher altitudes is thinner thus the measurement will be different for speed. TAS corrects for that difference and measures your actual speed across the ground.
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