Hmm, Ferrari kept him at Marussia for a second season and he almost certainly would have been in a better car (Sauber rumoured) this season, so they were convinced enough to continue .
Monaco does separate the good from the rather ordinary (though to a lesser extent to what it did), and while Max Chilton is hardly a good yard stick, he did lap him at Monaco last year, and how often does a driver lap his team mate these days?
I'm not convinced he would have be WDC ability, but I think he'd be good enough to sit in a top car in a Rosberg sort of way, and some do respond to a better car better than others. Look at how much Pedro Diniz improved moving from Forti to Prost and Sauber - he went from a no-hoper laughing stock to someone who could out-perform an (admittedly quite a way south on the down-slope) Jean Alesi, which however you look at it was a huge improvement. Conversely Alonso managed to show the world his ability in a Minardi. But Frentzen, one of the highest rated drivers in the history of F1 while in sportscars and for the most part at Sauber, somewhat crumbled under the pressure of getting a seat in the top car at Williams, yet was able to challenge for the title in a Jordan two years later. It's not always obvious how a driver is going to react to getting a top seat.