but how do you know how the Aircraft will perform when above the 40degrees
We do know how the aircraft will perform in the climb up to 50°C, both dual and single engine, because the manual has a chart giving the performance up to that temperature with no extrapolating involved. It's data in the flight manual, provided by the manufacturer.
Why have a chart giving the climb data to 50, if 40 is the limit for take off? Why would the manufacturer go to the expense of developing that climb data?
Edited to add: The SR22 take off data goes up to 50°C, and the preamble includes the note
For operation in outside air temperatures warmer than this table provides (50°C), use caution.
Difficult then to see how the temperature on the take off chart can be considered a limit.