The temps on the 5A/B cycle could have been lowered in the performance design phase to boost the disc and shaft lives (lower temps = longer life). While not optimal from an efficiency point of view, the A320 doesn't sit in the cruise very long relative to an A340 and so a higher core temp may not provide much benefit in terms of fuel consumption but it would significantly extend the life of the discs and shafts which are life limited by cycles.
Cycle life limits on disk and shafts are not very sensitive to cruise temps.
However, turbine blade life is
I will be interested in what Barit1 has to say on this