>isolating the sheets by coating them individually before stacking will limit the eddy currents thus allowing the device to operate efficiently.
A common fault in core fabrication are dull sheers on cutting the sheets which also exposes the SiC coating. But the shorting should only be on the edges. But SiC coating is also a rust protection which ought to have been done to coat the edges to prevent rust. A Kelvin bridge current source could be done to prove the microohm resistance difference between suspect uncoated transformer steel and edge shorted SiC coated CRGOS. Eddy currents may be detect by odd harmonics. Core saturation from fundamental voltage tend to be 5th harmonic. from my limited experience on MVA transformers.