Bar-sto makes some of the best after market barrels out there but they are expensive ($200) and you might easily have to wait 4-8 weeks for one. You probably can get a factory barrel pretty quickly and if you are not going to have bar-sto fit it then in some respects it isn't worth the money.
One thing you might not know is that Baretta high-caps are easily modified to work well in the xd.
You might want to check out:
www.xd-hs2000.com