Amazingly, I have already received those brand new inserts. Unwrapped the first one, and while loading a new roll of film found it to be MUCH tighter, and slightly more difficult to roll, meaning it required more force to pull the film across while spinning the take up spool. Shooting a test roll soon with the f/1.9 lens again.

I shot a roll with the f/2.8 kit lens, and it didn't seem to backfocus, but that's difficult to tell with a cheap development locally and a quick scan myself. Especially at f/2.8. This initially led me to think though that it was the f/1.9 lens. But that tension on the new insert really speaks volumes, giving it a try today.