Thanks for the inputs so far.
i just did a calculation for a Takeoff which seems to be obstacle limited with an OAT of 9 degrees.
Results:
V1 136
VR 158
V2 166
Flaps 1+F and Flex 46
As soon as I select 10 degrees OAT I get a different result.
V1 133
VR 152
V2 159
Flaps 2 and still Flex 46. This takeoff is not obstacle limited anymore.
So, you can not just blindly trust that your performance will not change with a change in temperature?