16lb is what works in my XDm with the 9mm conversion. I am also told by SA Customer Service that the XDm 9 ships with a 16lb spring. I assume the same rules apply to the XD - I could be wrong... My "assumptions" along with incorrect info have gotten me into trouble before...
The standard claw on my XDm .40 just barely grabs the 9mm case lip. I guess this is due to the design of the breach and extractor limiting the claw's reach and grab on the narrower case as you describe. I have not seen an XDm or XD 9's breach/extractor up close so I can't comment.
However, the extraction/ejection on my gun is very reliable with some brands of 9mm ammo. I don't have enough test runs to know which ones yet.
On the other hand, my XDm .40 with a 9mm conversion barrel will absolutely not cycle any 9mm ammo properly (FTEs, short cycles, fail to lock on last round) with an 18 lb spring. It will operate reliably with some brands of ammo with a 16 lb spring when all other variables are the same (mag, rod, extractor, ejector, slide, etc.). This same gun fires all .40 ammo I've tried flawlessly in standard configuration with the 18 lb spring.