I got a 9mm conversion barrel from bar-sto for my 2340 (Originally a 357SIG), and it rocks, it's as reliable as the real thing.
The thing is, the breechface was meant for the case head dimensions of the .40/357SIG, the extractor holds the round in place not by pinning it to the breechface, but by the hook itself.
Basically what I'm saying is, SIG wouldn't make that conversion unless the slide was changed with the barrel.
I personally would get the gun I wanted, but the conversion is still awesome for target practice.
I love my bar-sto conversion barrel, and wouldn't part with it willingly.