I tested it but going down to sea level was a little difficult in Smolensk. I just followed a river that went fairly straight on, and found that in Fw-190D_Late, with rad closed, pitch auto, WEP on, no cockpit (to see TAS), I got a sustained ~590Kmph speed. Still short of the figures in Il2Compare, but better than the 570 you mentioned.
Reaching that speed was, however, really painful in level flight, and the overheat message appeared quite fast. The engine went on for several minutes without losing AtA or RPMs, so maybe the message comes in too quickly. I never fly in this conditions when in combat, so I have hardly seen the overheat message before, and have no idea about it's accuracy.
About the La, I didn't have the time to test it in the same manner. Maybe I can today after work.
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