Both of them duplicate internally the algorithm used to generate the hair. Since Autodesk does the mental ray connection, obivously this is not a problem. I’m also pretty sure that when Pixar asked them for the algorithm, they were not in position to decline 
Would it not be possible to make a vray fur node that can be mapped in the same way as the maya fur description, making a conversion possible?We don’t really have any information about how Maya fur works. It would be a lot of trial and error to try and figure it out and we will still probably miss some cases. With that said, I know that people have successfully written converters for transferring fur textures from Maya fur to V-Ray fur.
Is there some way to optimise this to be able to use transparency without the 2500% increase in render times?Raytracing of transparent hair is always going to be slow. In any case, we are working to add support for the V-Ray hair shaders for the Shave plugin, which should help with this. They are much better optimized for V-Ray than the built-in Shave shading.
Best regards,
Vlado