Be careful when comparing the X & XA2 by core number. As you can see:

Xperia X:
Hexa-core (4x1.4 GHz Cortex-A53 & 2x1.8 GHz Cortex-A72)

Xperia XA2:
Octa-core 2.2 GHz Cortex-A53

While XA2 has 8 cores, it's 8 times the simpler less advanced A53 core, while X has a big-little cluster with four simpler A53 cores and two more advanced A72 cores. This could have some advantages in single core performance and also in the rather important "race to idle" (getting task done as soon as possible so that the phone can quickly return to low power states).

On the other hand, it might still not be really noticeable with real world usecases (maybe people already did some comparisons/benchmarks on Android ?).