I think that if the diameter of the threads is large enough and the stress relief before contouring properly done, that there is no bad effect on the target from turning threads at the muzzle of a button rifled barrel.
The reason that I say this is not some magazine article, but actual experience, and that of others. Before we installed the tuner that Jackie gave me, Don Nielson had tried an experiment at a match that I was at that involved threading the muzzle of what I believe was a Hart barrel. The experiment was a failure, and he removed the device, and finished the match with the threaded barrel, shooting groups that were consistent with what one would have expected from an unthreaded barrel. I don't know the size of those threads, but they may have been under .900.
Since the button rifled barrel that I used for my tuner experiment would get 15/16 fine threads, I figured that I would be OK. As it turned out, I was. Not to disrespect Mr. Calfee, but rimfire and center fire bullets are different animals, so what may be important to one, may not be for the other. Also, the Buckys, Beggs, and Stewart tuners all involve threading the barrel, and they have all been used to win matches. Lou Murdica has done a lot of work with the Stuart tuner, and I do not think that he would have kept threading those barrels if there had been a decrease in accuracy.
On a side note, back in the late 90's, I acted as a go between, between Lynwood Harrell and Lou Murdica, to facilitate testing of Harrel tuners on some of Lou's barrels. After that testing, Lou used one of the tuners on his heavy varmint rifle at the nationals. I asked him if he would have done that it the tuner was not an advantage, and he confirmed my thought that he would not have.
On a personal note, a friend fitted the first tuner that I tried (from Jackie) and stepped the barrel to make weight, and if I had had a lathe of my own, after the weekend that I shot it at the match that I mentioned, I would have cut threads and steps on every barrel that I owned. I still would today. The improvement was that distinct.
I will say one thing about changing barrels by tuning, or whatever. I would expect to have to rework my load.
Since that time I have received a couple of other tuners, and played with them some. This has made me a believer it having some weight at the end of a barrel, as well as some sort of rubber to damp vibrations. The differences are subtle, but they are there. My biggest problem is that none of them read flags.