I'd say that these players have all shown themselves to be capable members of the squad, I'm not saying that Fazio and Davies have been without error defensively of course but Fazio has demonstrated his dominance in the air and organization, Davies (even though he's not everyone's cup of tea on here) is much more preferable to Naughton playing LB and arguably has provided the competition that has had Rose upping his game, and Stambouli, generally I've liked him more each time I see him play, he shows good positional awareness, often quick feet with good control and doesn't hide like another French midfielder we have on the books.
I don't see any of these players being anyone's top signing of the season but I'm not sure what you consider to be telling about the performances of these players either.