The effect of frequency on a transformer is tied to the quality of the "iron" used in its core. In years gone by valve amplifiers had transformers in them that were essentially flat from 30 HZ to 15 KHZ, its just they were big and very expensive!
using a 60 HZ transformer on 400HZ, the higher frequency will result in higher iron losses. Material becomes lossy when its magnetic flux is alternated at this higher frequency.
Using a 400 HZ transformer on 60 HZ. There will be insufficient inductance in the windings, so they will consume a lot of current (off load) which in turn will saturate the core (core becomes non-magnetic).
This is why the voltage must be reduced, to keep the current down. So KVA is down (same current , lower voltage). Copper losses = i^2 X R, same R, same i, r= resistance of windings.
Efficiency is up because iron loss is lower, - lower frequency.
Frank