Going down in caliber makes no sense to me in this platform.
Perhaps looking at the bigger picture would make more sense? Maybe one of the reasons a person wants the XD-S in 9mm.....economy of scale in regards to reloading.
Let's assume I currently reload only for 9mm. (that is not true but it will work for this example) As such, I have a good supply of brass, small pistol primers, bullets, and powder.
Time to begin reloading for .45ACP for my newly acquired XD-S.
Oops....wrong primer size....go buy a new brick of primers.
Oops....wrong caliber bullet.....go buy a new box of bullets.
Oops.....no .45ACP brass.....go buy some once fired brass.
Hmmm.....powder....got lucky there, I can use the same powder.
Oops....wrong reloading dies.....go buy new reloading dies.
Let's see how that adds up....and we'll assume you can actually find the appropriate reloading components in the current market. Prices will vary....but the numbers I'm quoting were just pulled from the internet and/or local gun store pricing. They do not include any tax, shipping or hazmat fees as applicable.
1K primers - $38
1K bullets - $125
1K once fired brass - $95
Dillon 650 caliber conversion - $78
Dillon 650 tool head - $28
Dillon 650 die set - $64
Total = $428 + tax/shipping/hazmat
So....before I pull the trigger on 1 round of .45ACP in my newly acquired .45ACP XD-S, I will spend at least $460 in addition to what I just spent on the pistol.
As you can see, deciding to stay with the 9mm cartridge that I already load, at a rate of thousands per year, might be a fairly motivating reason to not jump into a new caliber.