As Aurick said, baking is the key.
Enable Draw > MRGB and Draw > ZAdd and merge all visible document layers into 1 layer.
Now you can perform a best render (without Depth Cue)and bake the pixols at 100% with Layer > Bake. This results in a document with the best render result, the depth preserved and a flat color material applied to all pixols.
The flat color material ignores lighting and shadows during both preview and best renders. This is excellent as it means much faster processing of other best render features such as Depth Cue.
Enable Best Render and tweak your Depth Cue settings. Then increase Render > Antialiasing > Size and Super Sample, in that order (Super Sample is basically multiple renders in one go).
If you still do not get enough in/out of focus bake the document again.
Hope that helps.
The only side effect is how the border of the document contracts as you can see below. However that can be alleviated by using a script to expand the border. The effect shown below is also fairly extreme in that I Best Rendered and baked 4-5 times.
Depth Cue Example