Before the Grammy performance was announced there were less than 700 registered here, and I would think that will definitely let you know the opinion of some Stewart fans particularly (or at least the ones who are registered and vocal, that is), but in order to sell out shows on a tour of this caliber it would take a cross section of a different sort to figure...a calcualtion that really, no matter how much information you have, will be really really difficult to predict in advance...this is why the dates have been released in the manner they were, to test the *general* demand and not self-saturate demand...we can know a lot know in hindsight but before the release of the big news, it was a different thing...the results of which seemed to even surprise Stewart and Andy (from what I gather from their own reactions in interviews since then)...
I do think that the forum has its own influence in other ways, though, even if the larger claims would remain unquantifiable...for example when Stewart replied in the poll that appeared here after the rehearsal announcement (the question about samples, the three of them, or do wop babes)...I think that the opinions here likely had at least some influence on it being the three of them...even though Sting had already announced it would just be the three of them on stage (but that option could technically include the use of samples or Andy and Stewart singing backup)...but seeing Stewart was taking singing lessons before heading to Italia, well, it seems that it will be the three of them with Stewart and Andy singing backup...something that's much more taxing than tracking the samples over and from Stewart's own comments not his fave choice but definitely the overall voice of the 'fans'...so I would have to say yes it did in some ways, as non-proveable of a claim as that is