Way back when the concept was coming to light I remember reading that it could be beneficial on revolvers if done correctly. It had to do with a bit of tightness in the bore where the barrel is threaded into the frame...kinda makes sense. It was not suggested for rifles for quite some time, as far as I can recall.
Numerous people have tested and reported on firelapping rifles and it seems the results are pretty much a mixed bag.
I do know one guy who told me the first thing he does with a new rifle is firelap it. Crazy, IMO.
God I am hating caliber threads more and more