exr is a 32 bit format developed by ilm ages ago, its industry standard as it has the most detail in it compared to other floating point formats like log luv.

The difference between luminance the eye can detect is around 4%. If the value step is greater than this you start to get banding and ugly format problems, most hdri formats have around 2-3% difference, but exr can hold 1% which is good for cgi as it can hold up well to lots of blending, layering and manipulation.

Exr doesnt have as high a dynamic range as other formats but its plenty for cgi. The best thing about exr is that it can hold hold multiple passes like occlusion, UV, world space normals, z depth, reflections, refractions, shadows, ID whatever you want. So its really good for compositing, unfortunately maya 2008 doesnt support that functionality of the exr format, im sure you can do it with mental ray but its too much to be bothered with, so i usually just output all my auxillary information as seperate sequences.

The artifacting your getting looks like its due to your refractions being undersampled, i would really just do those blurs in post. Its a bit like motion blur, it looks nice as its truely 3D in mental ray but it really isnt worth the render time. Render out a reflection and refraction pass, and an ID pass then you can blur and tweak in post.

With your bsp, for a scene like this i would jsut use defaults, its not a heavy scene, its important if you have busy scenes as there is a chance of maya crashing if it tries do deal with too much at onces. For example if it tries to render an area at once which has hundreds of polygons and textures in.

Do you have any compositing experience? Or have any compositing packages, if your on mac buy shake, its cheap now, or if your on PC AE is the affordable option.

If your rich, i would go for nuke or digital fusion are great out the box.