Juraj, I remember some of my old test with VRay (opacity, opacity with filtering off and real 3D model leafs). With opacity map, the trees, obviously, are more light. With real 3D leafs the rendering time with VRay was much better. But, when you have to render a forest, cause out of RAM, I had to use opacity. Same in Corona: 3D leafs are better, because with opacity map, Corona is slower. So, you can understand why I'm so shocked! Now, in VRay, you can use opacity map without problems.
Yes Ondra, I know veeery well. And, if you look my old gallery
EXAMPLE (entire forest with trees) you can see how many project I did with VRay.