Sometimes you can double conversions with the same traffic
I can vouch for this first hand. I spent 4 months optimising my main site for the visitors, testing and implementing better calls to action, making pictures more intuitive, repositioning data and improving the layout and my conversion ratio nearly doubled.
One of the best bits of advice I ever had was: "it is x times easier to sell to an existing customer than it is to find a new one". (I think x was 5 but I can't remember lol).