Based on stonewalrus' question, I changed to the thinner backstrap on the XDS9 and tried it out this morning. Groups more centered but still 1-2" low @ 7 yds. I'm going to stay with it for a few more sessions but it does not fill my hand as much.

Backstrap for sure. I will try the thin one when I get a chance but expect the same results. Guess I will eventually be looking for a taller rear sight.

Why don't you get a shorter front sight? Nicholson makes 'em, and they can be got at most any hardware store....

It would probably be easier to put on a higher rear. I have not been able to find either specifically for this issue with an XDS.