These should all render identically.

CASE 1: no opacity set on any layers
CASE 2: opacity set to .99 on overlays but not on basemap
CASE 3: opacity set on overlays but not on basemap, zIndex option set to 0 on basemap
CASE 4: opacity set to .99 on overlays but set to 1 on basemap
CASE 5: opacity set to .99 on all layers
CASE 6: opacity set to .99 on 1st and 3rd layers and 1 on middle layer