So if there is that much difference in backbore thickness, how do you know if the cups and rims are machined correctly? I wouldn’t touch a Bach mouthpiece.
I went to Denis Wick Heritage series mouthpieces and would highly recommend them. The Army Field Band trombone section played on them at one time and may still. Sizes are similar to Bach sizes and numbering but absolutely consistent. With all of my students the first thing we do is get them on a Wick mouthpiece.