As the title says...I have a Springfield XDM .40  3.8 Barrel gun that I'm really interested in buying a .357 SIG  Conversion Kit for...Does anybody know where to buy the kits?  Is this conversion worth the time and money?  Anyone had experience with this and shooting better  etc.

Bar-Sto and EFK Fire Dragon both make .357 conversion barrels for the 3.8" XDm-.40.  Bar-Sto advertises theirs as a "semi fit," which suggests that it might take a little more work than just dropping it into the gun, EFK doesn't really say.  

As to whether or not it's worth it...  as was mentioned in your thread in the Glock forum, .357 Sig is not the cheapest or easiest to find ammo out there.    So whether it's worth it is going to depend on how much you want to shoot some .357 Sig.