I went back and more rigorously tested for audible differences between poly-gauss-xl and xla. Any differences that I initially perceived were the result psycho-acoustic bias. I was NOT able to pass a blind ABX test. A comparison of the upsampled PCM files revealed high frequency artifacts in the audible band, however, they were not audible at listening volumes and only became audible to me at ~20dB+ amplification. It is possible that more severe apodizing errors may be present in other tracks, but there was essentially no audible difference in the tracks that I tested, despite each having 100s of apodizing errors.
Here is my full analysis: Apodizing vs Non-Apodizing: ABX & Delta Waveform Comparator Results (no audible difference)