Lepanto resulted due to an effort to save Cyprus for the Venetians, which the Spanish et al agreed to in exchange for Venetian support in North Africa.
So Lepanto wasn't really about preventing the Ottomans moving West, and the fleet was assembled too late to save Cyprus.
Don Juan came across the Ottoman fleet which had been raiding Venetian possessions along the Adriatic coastline, and the rest you all know.
If the Ottomans had won, it would have been a disaster for the Allies, but they would have been able to rebuild their fleets just as easily as did the Ottomans.
While the morale boost wouldn't have been had, I'm not sure the Ottomans were in a position to follow up.
If they had, their ambition at the time was to consolidate control over North Africa, so I would think any offensive would have been directed at Malta and then Morocco.
That could certainly lead to overextension - note that at this time the Ottomans had just mounted a serious campaign against Cyprus and the attempt to take Astrakhan and build a Don-Volga canal, and I'm not sure the forces to go after Malta could have been assembled fast enough - and the panic that ensued from Lepanto I would think would lead to continued cooperation of the Allies in defense of Malta.
So I think the most likely result would be a minor gain for the Ottomans in the way of pressure lifted, or no difference, and possibly the Ottomans being prompted to reach too far.
In this period, the large Ottoman inferiority in manpower and resources was starting to be felt, as large-scale conquest and the booty thus obtained were drying up, but the Empire didn't seem to recognize the glory days were over yet.
To be honest, I think it is a miracle the Ottoman Empire managed to take AND hold the territory it held. Its manpower was far from unlimited, and it already had to devolve much of the local authority to the regional rulers as it was, to where Constantinople's writ on some of the outlying Mediterranean provinces was almost theoretical even near the Empire's zenith.
A victory at Lepanto may actually be worse for the Ottomans in the long term, as they would have a rather hard time expanding further West and being able to keep their conquests. In other words, they are almost overstretched as it is. Further conquests may be made, but are unlikely to last, and may actually cause earlier fall of the Sublime Porte due to the amount of resources and manpower needed to keep them.
I would definitely like to see Abdul's opinion on this, as our resident Ottoman expert. What do you think, Abdul?