Maybe some "revolutionary" votes from the left
I mean : votes for Sarkozy to prevent a president Bayrou from dismantling the PS and the Greens.
At least, there would have been a "bonnet blanc/blanc bonnet" campaign from the "real" left, like in 1969.
The Bayrou momentum was too early in the campaign and the big margin of Sarkozy in the first round would have been the same.
Plus, I think even more FN voters would have voted for Sarkozy (I mean leftist voters of the FN, who hate the "centrists", the "PC" politicians, the European federalists, etc).
So I think Sarkozy would have still won, even though by a short margin (probably like Giscard in 1974 or something around 51-49).