ad should implement arnold in si?
that would be the worst thing that can happen.
look at mr and what ad is doing with it at the moment. support for mr is really bad because you have to go to ad. on the other hand, mi is not responsible for the integration.
arnold support is great and you have fast bugfixes. bugfixes for mr means you have to wait for a x.5 version which you only get thru subscription (except show stopper bugs, then ad is great enough to give a hotfix away). the whole situation is a bad joke at the moment. so lets hope this will never happen.
that would be a bad thing if the hair consist of flat patches only and not thick curves/geometry. because the lighting and shading will never look really good, except the shader uses some tricks to go over the shading problem. would be great to get some more informations here.
and in general, transparency on hair kills rendertime for all renderer. this is one thing why mr hair is slower when render with hairshader, the transparency is enabled, deactivate it and it renders 2,5-3 times faster. you have to handle the transparency different for hair rendering. nvidia arc(mi) is doing this constantly, newer version should bring here better performance.
so if this is a problem in vray, we should start a wishlist thread vor vray asap.