Dennis,
Your point about the diffraction energy in the sidelobes relative to the central peak may be a good one. I combined my two ZEMAX predictions into the one graphic attached, all plotted at the same X-Y scale, and it is easy to see that the Bahtinov sidelobes are brighter than those produced by the Y-Mask. However, the Y-Mask blocks only a few percent of light to focus in contrast to the Bahtinov, which blocks at least 50% of the pupil. The Y-Mask sidelobes are fainter, but the overall image is brighter by almost an f-stop. The Y-mask sidelobes are also narrower than the Bahtinov, allowing more precise location of focus to be determined if they can be imaged well.
Guess it gets down to direct empirical photographic and visual comparisons in the field rather than just studying numerical analysis. If the Bahtinov is still superior in locating best focus to the Y-Mask in real-world practical applications, then the Y-mask still goes down as a good idea but just not as good. They could also be a wash, depending on the mask geometries used, the brightness and spectrum of the star being used, and the degree of turbulence present.
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