Yeah, about 0.002”. I’ve recut the jaws on my original 8”-3J. It is pretty well right on. On a scroll chuck, the jaw position is determined by the face scroll, that scroll must have a little bit of clearance to the chuck body (if it didn’t, then it would not turn), where that clearance lands will introduce some variation in where your part ends up relative to the perfect center position. It is a common practice to dial in the part in your 3J, tap the part sideways as you snug the chuck (dial it in almost like you would a 4J independent chuck). Often you can get it pretty well perfect.
6J you ask? I have the Bison set tru offering, 6”. Obviously being set tru I can dial out the run out (also being a scroll chuck, there is the same issue with play in the scroll). Certainly, it is a very nice chuck, however I agree with the comments above about it not being really any more precise - sure it is good, but not magic. It is no better than the 60 year old Roem(?) that came with the lathe. I like the 6J, I use it about 25% of the time. Lots of times it is not the best option - it does not close down as far, the chuck jaws are physically larger (sometimes you can’t get into places), it just doesn’t feel like a heavy duty item. As others have said, it is good for thin wall items. I like it when holding a narrow part and maybe only 0.1” into the jaws - like just grabbing with a fingernail hold.
The 6J is great, sometimes it is just the ticket for a certain job - which is true for any of the various work holding options that you can tie onto the spindle.