Icarus,
As someone else said, pretty good chance that the V speeds would be pretty similar, but when it comes to Flex, there would be a big difference in Flex temperatures.
So, if you took off from an intersection 400M up the runway than what you calculated figures for, and you lost a donk at V1, then no, Flex power will not be adequate.
Flex is just the Airbus way of saying Derate. There's not just one setting.