You raise a good point, i.e., whether the fusion to the root would be more easily attainable had stainless test plates been used.
A similar situation arose on another project and once again I administered the fillet break test on 304 stainless base metal. In this case the test material was only 3/8 inch thick to replicate the project conditions. Once again, the results were the same as had been in the case where carbon steel plates were used. The fabricator did not have E308-15 electrodes and proceeded to fabricate the parts using E308-16. Because the parts were already fabricated before my involvement, the engineer accepted the installed components "as built" based on the fact that the welds were 3X what was required to transmit the design loads. The length of the welds were based on the need to "seal" the joint versus the need to transmit loads.
The point is, my experience indicates the flux covering on EXXX-16 and EXXX-17 results in very poor "penetration" characteristics.
This brings us back to my position that it is a fallacy to believe that because a welder passed the groove test, he/she is "automatically" qualified to deposit fillet welds. In this case, had the fillet test been omitted because the welder had previously passed the groove test, we wouldn't have known the extent of the welder's skills problem or the difficulty as a result of the electrode classification (flux covering).
Best regards - Al