"Since the Saami specs for land and groove diameter are identical for 9mm, .38Sp, and .357 Mag, it seems to make no difference in blowby. At least it isn't a problem in my three J-frame conversions."

The theoretical difference is not in throat and barrel dimensions, it is the 0.4" jump through the .38 chamber that might let the 9mm bullet wander around. You and other users don't report any failures that might have caused, so it stays theoretical.

I had a couple of 686s cut for .38/.357 clips and did not find them a great advantage. The long skinny .38s flop around a lot more than short stubby 9mm or .45. Maybe I just did not have the right technique. IDPA rule changes obsoleted any clip gun smaller caliber than .45 anyhow.