I am reporting what the actual experience on the A318 has been - forget theory
I've been flying the A320 series since 1999, so my statement is not based simply on what is written in FCOM 1.
FCOM 1.28.10
FUEL RECIRCULATION SYSTEM
Some of the fuel supplied to each engine goes from the high-pressure fuel line in that
engine, through the integrated drive generator (IDG) heat exchanger (where it absorbs
heat), to the fuel return valve, and to the outer fuel tank.
This operation ensures the IDG cooling when the oil temperature is high or when at low
engine power.
The FADEC controls the fuel return valve.
If the outer tank is already full, the fuel overflows to the inner tank through a spill pipe.
I don't ever recall seeing outer tank temeratures less than inner tank temperatures in flight.