I don't have a 9mm, but I do have a .357/.38 special adapter (Chiappa 8" X-caliber) for 20 gauge shotguns.
I've tried it only a little in my NEF Pardner. I didn't get groups as good as yours. Whether it's because I wasn't good using a front bead or because it's just not accurate I can't say.
The theory of adapters is intriguing; the practicality (for me)... I don't know. I initially thought that the option to both .357 and .38 special ammo out of a firearm that already has the versatility to shoot bird, buck, and slugs seemed pretty great. In reality slugs, .357, and .38 specials all had different POI, and I don't know if any of them really hit at POA. Having only a fixed front bead limited me from making any helpful adjustments.
So it would only be a very short range option for me. In which case, I don't know if there's anything I could do with the .357/38 adapter that couldn't already be done better with bird or buck shot (except carry smaller/lighter ammo...which may be the point for some people).