So how do you separate the player and the system? For me, it is extremely difficult to separate them. A lot of people say he has to perform for Argentina, he has to drag them to a World Cup, because it's that that defines a player, I don't necessarily agree with that, but what he has to do for me to be comfortable in saying he is the greatest ever player is do it for a team who is not completely dominant. That might be Argentina, it might be another club, or it might be Barcelona when they drop down a few levels.
I wouldn't expect him to move, and it's certainly nothing one can criticise him for, but it does leave a question mark there if he only does it for a completely dominant side.
I've already said that I think it's folly to even compare Messi to other players.
I'm directly staying that I believe they would score as many goals. Rooney scored 34 in an injury hit season when the side was built around him with the likes of Carrick, Fletcher, Gibson and Valencia creating for him, I would expect him to score a lot more goals if he had Xavi, Iniesta and Henry/Villa/Ibra (depending on the season) providing the goals for him. van Persie has scored 28 goals (I think) this calendar year, and much of his service has come from Arshavin, Song, Gervinho etc, and there are still 2 months to go. If he had those amazing world class players who have grown up together dominating every game and providing for him, why on earth wouldn't he score a lot more? Ronaldo scored just as many, if not more than Messi last season, in less appearances, with inferior (though very good) team mates, I'd be very surprised if he didn't hit even more in that side.
It's possible, indeed likely, that the team would be adversely effected by this, Messi has come through the Barcelona academy and he knows how to play with those players, but how do you separate that from how good he actually is?