I think if Tory enters the race, it would hurt Rossi, Ford and Smitherman alike and help Pantalone.
Rossi is the most pure "Yonge/Bayview Corridor" candidate to date. I would guess his best showing would be in Ward 25 - by far John Tory's best ward in '03 (and won of two won by Jane Pitfield in '06). His candidacy so far has been a flop, and he might be the most impacted by a Tory candidacy.
A lot of Ford's vote to date seems to be Conservative vote-parking, so a good deal of his support would evaporate with Tory. But to what extent does a populist, "Reform Toronto" constituency - fed up with crime, taxes and bureaucrats, etc. - exist? John Nunziata ran that sort of campaign in '03, but he only obtained 5% of the vote.
And with Tory in the race, Smitherman loses ground for two reasons: he can appeal to centrist to center-right voters who are currently backing Smitherman as a "winner", and furthermore with Tory rather than Ford being the main right-leaning candidate, there will be less strategic voting among progressives which helps Pantalone - as Tory can't be painted as "scary" in the way Ford is.