I found that increasing the infill overlap helped the undersides of spheres. Default is something like 15%, try increasing to 30-35%... 40% didn't help much more for me. You may need to decrease the extrusion multiplier by 1-2% to compensate for the increase in overlap. YMMV.
Well, I started a build with a dozen balls to test the Thermal Theory - stopped it after they had built enough to check. They don't look any better than the single - so that theory is busted.