Higuain, Hamsik, Callejon, Reina, Maggio, Mertens, Pandez, Dzemaili, Insigne, Armero, Inler and Albiol would regularly start for every club outside Barcelona/Madrid/At Madrid in La Liga and some of those players would get games for those clubs as well as other top clubs such as United, Arsenal, Bayern, Dortmund etc.
Let me just start off by saying that player by player comparisons aren't that good of a way to determine how good a team is, and shouldn't be the final reason to deciding why a team is better than another team. Nevertheless, since you brought up all those wonderful players, let's do it anyway.
I can guarantee none of those players would start for Arsenal, maybe Higuain but I still rate Giroud over him. United I wouldn't judge as a benchmark for quality, as they regularly start Cleverly, off-form Valencia and Young, and most of their defenders are currently playing like crap. For Bayern, even Higuain would have a hard time getting on the bench, there is no way at all any of those players deserve to start for Bayern. Dortmund their attack is better, Lewa>Higuain, Reus>Hamsik, Aubameyang>Insigne, etc., maybe a CB could get in to rotate Hummels, but otherwise, no chance. Let's keep listing clubs until we find ones that would regularly play Napoli's players, shall we? Atletico might play Higuain in place of an off-form Villa, and find some use for Hamsik, but other than that, nope. Real should obviously have kept the Argentine and played him instead of Benzema, but the other players wouldn't touch Real's starting XI. We would probably play Higuain when Messi was tired, and could maybe use Hamsik as a bench player instead of Tello, but none of the others make the team. Manchester City could probably find use for Hamsik or Insigne on the left wing for more width, I rate Higuain higher than their strikers, perhaps Fernadinho could be replaced, and on current form I would pick Reina over Hart, but other than that no one.
Chelsea, Higuain would probably start, but he might still struggle as strikers have recently done for the Blues. Hamsik might come out ahead of Schurrle, Willian and so on for the bench spot, but he wouldn't start. Now we get to Liverpool, and here we see some changes. Higuain is too good not to incorporate in the XI in some way, Maggio is probably an upgrade over Johnson, Liverpool's midfield could use some reinforcing and I would pick Hamsik ahead of Moses. Everton would trade pretty well with a combined starting XI of the two clubs having half of each team, maybe slightly more of Napoli's. At the moment, Southampton is playing well, but their actual players aren't that good, and Reina, Higuain and Hamsik should all have a comfortable time getting into the starting XI, as well as some of the midfielders you mentioned. Let's go to France, shall we? Only two good teams here, and those are PSG and Monaco. For PSG, Reina would probably start, Maggio could give some competition to their RB, Hamsik would probably be better than Lavezzi or Pastore, and Higuain would consistently get on the bench and be rotated, but that's about it. For Monaco, I again could see Reina, Hamsik, and Maggio playing, and maybe the system is changed to include Higuain.
There's a few players on Napoli's squad that are consistently in contention for a spot, but the teams that would start those players are not looking favorites to win any major trophies, unless you consider the Ligue 1 title a major trophy. The title contenders, the likes of Barcelona, Bayern, Dortmund, Madrid, City, Arsenal, they wouldn't take any of Napoli's players for their starting XI, and that's what separates them from being a good team. Wait, I just realized, I forgot a whole league's top teams! Serie A, of course, how could I forget! Let's start with Juventus, shall we? Although Tevez hasn't been playing badly, 3 goals in 8 games isn't the best return, and Higuain should start over him. Hamsik is better than Vucinic, although that's not really saying much. Maybe Asamoah could be replaced by someone, but that's about it. Still, that's more than for most other top teams, right? Now let's go to Roma; Higuain could be incorporated in the starting XI, Hamsik could compete with Florenzi, Maggio could be better than Maicon and Reina should start.
What I noticed from doing this is that Napoli's players are instantly in contention for starting in Serie A, even with the first place team; with other leagues, the starting XIs of the 1st and 2nd place teams (Ligue 1 excepted) are all stronger than Napoli's. Even if you have a different opinion from me, and rate some of those players higher than I do, you can still admit that only one or two of Napoli's players would start for us or Arsenal or Atletico or Bayern, while three or four would start for Juventus and Roma; that's why I think Serie A is a weaker league and why Napoli is only a decent team. They have a few good players that could start for top clubs, but for most of them it takes until 4th or 5th to start featuring in the starting XIs (6th or 7th in EPL), which is in my opinion not enough to be considered good. And yet they sit second in the Serie A table; wwould you really see them doing the same in any other league, maybe even in Ligue 1? I personally wouldn't.
