You're right, Simon. Way over-estimated. I should know better than to answer at 5:00 a.m. with no coffee yet.
A paternal half-sister should, if the mothers are unrelated, match a male's xDNA not at all other than by chance (or identical by state). To try to dig myself out of that hole...
In males, the X chromosome is hemizygous, meaning that we have only one, but that it's joined at the pseudoautosomal regions with the the Y chromosome. So for males the raw data from most microarray autosomal tests display all alleles on the X chromosome as homozygous, i.e., AA, CC, GG, or TT (assuming no loci are no-calls), even though there is only the single, haploid X.
For matching purposes, our DTC testing and reporting companies look at half-identical regions, HIR, so any one of the two alleles from one kit corresponding to any one of the two alleles from the other kit would be considered a match at that locus.
The X chromosome undergoes recombination during meiosis in the mother, just like the autosomes. A male child gets his X chromosome entirely from his mother, but it might be anything ranging from inherited in whole from a single maternal grandparent, to made up of typically no more than five segments inherited from a combination of both maternal grandparents. A female child receives one haploid X intact from her father, and one haploid, recombined X from her mother.
In the case of paternal half-siblings, then, the daughter would carry the father's intact X chromosome, but the son would carry his mother's recombined X. The two should never have an actual X chromosome match unless the two mothers were closely related. Any reported matching would be in error, brought about by the fact that the male's X is hemizygous; that our microarray tests examine only between 15,000 and 30,000 loci of the X's approximately 156 million base pairs; and that by-chance results in the female's allele pairs showed enough HIR matching loci to be interpreted as a contiguous segment.
Consequently, yes, full-sibling sisters will always present as sharing 100% on the X chromosome because they each inherited the same intact X from their father. Male-male and male-female siblings will match only on the haploid, recombined X from the mother. Effectively the possibility exists for the rare but not impossible situations of zero matching between brother and sister up to a son and daughter both inheriting the same intact X from the mother; more probable is that male-female siblings will always display significant X chromosome sharing, but the amount will be highly variable.
Ah. Coffee...
<cough cough>