FE Hoppy: Here is my take on what I think is being asked....
The OP correctly identifies that with a TOGA takeoff at 4C and another at 30C then the thrust used will be the same albeit with a lower EGT due to the greater air density and lower fuel flow at the lower temperature.
Now, step to the FLEX case. At 40C the FADEC computes the N1 setting as if the air temp. was 40C. OK so far?
Ignore the TAS issue.
He wants to know if the FADEC calculated N1 for 40C will benefit more from the 4C air that is being taken in (ie. more dense) or just from the 30C flat rated air density.
My question to you is this: when using a FLEX setting does the FADEC actually compensate for the air density differences between the ambient or is it simply programmed to work using the top end of the flat rated values?
This, I think is what is being asked, not the margins inherent with the IAS/TAS margins.