While I'm not sure about the flight model in reference to speed, climb or turn since I don't have enough data (especially for the last two, finding speed numbers is easier) and also are not entirely sure what all the numbers are in game, I am very certain the fast overheating for the German planes is not historical.
I actually remember seeing links to test/technical data for the engines heat produced and the radiator effectiveness on this very forum (But I sadly can't find it right now). If I recall correct the data there showed that the full open radiator would actually absorb more heat than the engine produced in a climb.
There was also a very big discussion not so long ago about the MW50 boost, that should have shown to everybody reading it, that full power flight with MW50 boost enabled on WEP power setting was officially allowed for 10min at a time and could be sustained for a total of 27min (after which time the MW50 fuel would run out) with 7min durations in between at lower power settings. The problem after 10min also seems to be not so much the heat but rather the stress on the engine produced by the high power setting. Military power was allowed for 30min at a time.
A lot of the data can be found somewhere on this page:
http://kurfurst.org/
I really see IL-2 as a simulator and would like to have FMs as accurate as possible, but with the current state of some planes FM I would settle for a faster "closer to reality than it was" fix rather than wait around for a very long time for a "very very close to the actual plane" fix.