Just did a quick test and here are some of my impressions. Please treat these as impressions and not necessarily real proper tests.
Same HDRI rotation, same model, same materials, same VFB settings. The camera angle is slightly off, that is why I'm saying its not a proper test. Still, you'll see that the conclusion is the same regardless.
- Opacity maps noticeably reduce the speed in Corona for C4D.
- Opacity map sampling doesn't make much of a difference.
- Opacity map clipping (filter, clipping) doesn't make much a difference.
- Corona Bitmap does make a difference although the performance disparity between 3ds Max and C4D is quite large. Example scene needed 50 seconds to get to 12 passes in C4D and only 18 seconds in 3ds Max. The difference in ray performance is 10-7 million (3ds Max) versus 2.5-3.5 million (c4d).
Stats:
3ds Max with Opacity maps: 18 seconds (10-7 million rays)
C4D with Opacity maps (+Corona Bitmap): 50 seconds (2.5 - 3.5 million rays)
C4D with Opacity maps (without Corona Bitmap): No idea, takes too long. (Only 800k rays)
3ds Max without Opacity maps: ~11 million rays
C4D without Opacity maps: ~11 million rays
As a note, turning off opacity maps in 3ds Max nets about +1 million more rays cast, in C4D it boosts them up by about +8 million. To keep it simple, there is performance parity between the DCC applications if there are no opacity maps involved. As soon as they are, the C4D implementation hits a major brick wall.
Another funny thing I noticed is that there is visual difference in the VFB contrast / exposure levels even when using the same HDRI. Not sure why but I'm just putting it out there.
edit:
The VFB difference is actually due to 3ds Max's default color picker not being sRGB while in C4D it apparently is. Values match better (although not identical obviously) if one switches to 3ds Max + Corona Color Picker. Just a heads up! :)