I don't understand your question. What do you mean by gain? The only fault I find with the ContourPlot is that the 1.0 contour is a little ragged at the left hand end. Try

ContourPlot[x (z - 1) + (1 - x), {x, 0, 1}, {z, 1, 2.5},
 MaxRecursion -> 4]

You might also try increasing the PlotPoints.

Even though ContourPlot is a 2-dimensional plot it can take a 3-dimensional PlotRange. Sometimes some areas of the plot will be White because the function value is outside the third plot range. So you could use:

ContourPlot[x (z - 1) + (1 - x), {x, 0, 1}, {z, 1, 2.5},
 MaxRecursion -> 4,
 PlotRange -> {{0, 1}, {1, 2.5}, {0, 1.6}}]