Pearce Grips make a magazine extension for most XD pistols to include the XDs. http://www.pearcegrip.com/
I put Pierce extensions on the 9 rnd mags for my XDSC almost as soon as I bought it.They don't stick out enough to change the way my XD patterns and make a big difference in making my grip more secure and consistent. I recommend them highly.
I added them to my wifes 9SC and she loves them. The ones I used are a little longer, but add 3 rounds to the capacity of a 9mm, or 2 rounds to a 40. JB
Here's a good video on putting the Pearce extensions on your magazine. Shows the 40 cal but applies to all. This is so your mag spring doesn't shoot across the room when you remove the end cap. [ame]http://www.youtube.com/watch?v=_apRnuUT3f4[/ame]
I just got a Pearce pinky extension for my XD .40 subcompact and it's the best of both worlds (a fullsize-ish grip without extending the handle). Springfield maybe should have just included something similar with the smaller mag to begin with. In any event it's the perfect accessory unless you have small hands.
I ordered them just out of curiosity - My XD .45 doesn't need them because the full size grip is long enough w/o the "pinky" extender. They look cool on the gun tho