I'd still be all in on Stamkos. Matthews or no Matthews, I just don't see any reason to pass on a young talent like that.
As Patrick O'Sullivan put it on 590 this morning, there's likely no way that Stamkos would want to come to Toronto now and ultimately (or very shortly) be the #2 guy behind Matthews both on the ice and in public opinion.
I don't see why that would be the case or how he could think that so likely unless he actually spoke with him about it. Plenty of star players have not been concerned about being surrounded with other star players. I can't imagine he's that full of himself that he would scoff at playing somewhere simply because there is another highly touted player there.
And there would be a very obvious way for Stamkos to show that he should be seen as #1...outperform him. But I can't see a guy from Markham, ON, playing for the Leafs and, presumably, lighting it up on the Leafs, not being revered in Toronto.
EDIT: And really - the guy is 2nd in goals per game since entering the league, 2nd only to perhaps the greatest goal scorer in NHL history.
If the Leafs could get him for under $10M per year I'd do it. It will be years (maybe even 6-7) before there are any sort of cap issues.