Hi,
often I need metallic car paints, not for cars, but for train interiors and objects like the waste baskets (like known from DB trains). I thought about it how to get it and I see two ways I can go now:
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render an image twice based on different materials and combine them in Photoshop or
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add an offset surface with a semi transparent glass/plastic
Here a quick test with an old model I found here. My results per offset surface are:
metal material only
metal + offset surface → some parts looks not like the right mix, caused by bias of the calculation
To avoid the bias error I scaled the model by factor 10 (window material changed too). Now it looks perfect. 
The result isn’t 100% stable yet, maybe it helps to fine tune the offset surface distance. It makes fun to play with the transparency of the red plastic coat and the roughness of the basic metal object. So, it’s quite easy to mix between metal and simple diffuse color.
I hope this test helps some users as workaround, shows the great potential of metallic effects for Enscape and is a little motivation for the team to add a metallic shader for Enscape. The real time performance is really good, no problems can be seen. A basic metallic shader could be an eye catcher.
-Micha
