HDR won't make a bit of difference. The most important aspect of these materials is in the reflections which has nothing at all to do with diffuse lighting. Poser IBL is important for getting the diffuse lighting to be believable. For these types of materials, you need to get the reflections believable.

In the Fresnel tute I talked about the importance of the environment that is being reflected. If you don't use a sky dome or sphere along with some other objects to interrupt the visibility of the sky, the reflections will produce nothing interesting. When you look at car paint, glass, chrome, etc. it is all in the reflections of the environment and in the specular reflections of the light sources themselves. IBL does not produce any specular reflections at all.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)