p.s. increasing the irradiance to 100% actually seems to speed up the "gather" render. increasing "gather" samples to 8 also helps, so it may be analogous to photon mapping. a quick render of the poser lo-res ball (with simple_colour node in both diffuse and alternate diffuse) above the ground (with gather node in ambient) showed that the floor demonstrates the "gather" effect with one lite at 0.000%. after deletion of the lite (no lites in scene), the floor still demonstrates the "gather" effect.