First of all, everywhere I said "offset breech" I should have said "offset chamber".
Second, I found one source (here on THR, actually) from a person who wrote to KKM about conversion barrels and was told that there is no offset.
https://www.thehighroad.org/index.php?threads/conversion-barrel-conundrums-help.743267/
"KKM has courteously replied to my question on this, stating that extractor change is not necessary and, that there is no "offset" in the manufacture of the conversion bbls."
Also included in that post was a response from Lone Wolf who suggested that an extractor change wasn't necessary but would make sense for a permanent conversion.
"Similarly, Lonewolf rep. replied that changing out the extractor is not necessary but would be fine if the conversion is permanent and dedicated to shooting only 9mm."
I also found some posts on Glocktalk (which I will not link here because sometimes their links do strange things) that showed measurements of a Lone Wolf conversion barrel with no significant offset (possible offset of about 0.0015") and another post with similar measurements of a Lone Wolf conversion barrel showing no detectable offset.
Also a post on Glocktalk claiming to have information from another person who contacted two conversion barrel manufacturers (without saying which ones) who both stated there was no offset.
Also another post on Glocktalk from a person claiming to have personally contacted Lone Wolf and Bar Sto who both said their conversion barrels have no offset.
There is a review where the reviewer claims the offset existed and I found it referenced more than once in the searching I did.
https://concealednation.org/2013/10/review-lone-wolf-40-9-conversion-barrel-for-glock-27/ The reviewer makes the claim about the chamber being offset but provides no information as to how that conclusion was formed. No measurements, no citation from a manufacturer, nothing at all. I don't think this is where it all started, but it does seem to have legitimized the claim somewhat although without providing any evidence to support it.
I've also seen a number of posts on various forums where various people claim the offset is there but then, when challenged, never provide any information/measurement data/citations (other than perhaps the review mentioned above) to confirm the offset when asked for evidence.
Anyway, that all seems moot as, in most cases, it seems that neither an extractor nor ejector change is necessary for satisfactory reliability in a range-use only gun.
I think everything points to going with the proper magazine and a conversion barrel for range use only with other changes only required if reliability is an issue or if the conversion is intended to be permanent or for self-defense. In fact, for range use only, you might get away with using the original mags. The feedlip spacing is a different, but maybe not enough to cause significant problems.