You can use the DMC sampler as global quality control, for example:

For lower quality/higher speed:

• increase the Adaptive amount
• increase Noise threshold
• reduce Minimum samples

For higher quality/lower speed:

• decrease the Adaptive amount
• decrease Noise threshold
• increase Minimum samples

These controls have effect on everything: DOF, GI, glossy stuff, Area light etc.

Color mapping mode also affects render times and sampling quality, since Vray tries to distribute samples based on the final image result.

The images below have the same subdivs for DOF (10 subdivs), GI (50 subdivs, Direct method), glossy reflection (50 subdivs, no interpolation), area light (10 subdivs) for all images. Notice how changes in the DMC sampler parameters affect the image quality and render time (click the images to enlarge them):