The batching that Disney is doing is a very recent direction and not something that is common - PBRT serves as a straight forward reference implementation.

You are right though that it would be fascinating to see the speed difference. My guess is that it would be substantial.

Even compiling PBRT with Intel C++ instead of Microsoft's C++ compiler gives a %30 - %50 speedup, likely due to reordering instructions for better memory coherency on a granular level (batching rays would give better memory coherency on a very broad level).