I just found that Storm Lake Barrels makes a 40S&W to 9mm drop in conversion barrel. This looks like a great option for shooting a lighter caliber out of the XD40 frame. I've heard great things about Storm Lake barrels. Stainless Steel, Match Grade and USA produced. I also found EFK Fire Dragon Barrels as well but they were a bit more. I might try and get the Fire Dragon with a set of ports later on. I've quickly come to really love the XD frame, it's like Lego blocks build what you want.
Seeya
Mark