Attached Link: Second test

I did another test render above:   ( 28 minutes on "preview" setting / 6 hours on normal )

I used 4 default texture types with HDRI. Light/atmosphere was stock settings. It seems transparency and reflections was handled well on the preview render setting. Original image was 3000 X 1688 and downsampled to 800 X 450. Obviously, you'll need to scale up if you need a bigger downsampled image.

For the shadows, you were right. Increasing the light/shadow quality does help alot. You don't need to do it if you're using "busy" textures for the surroundings. Maxing up the settings jacked up the render time abit but it's still significantly faster than a final setting render. So I guess you can go crazy on the quality settings if you're planning on rendering with preview.

Post was done with CS2, resized with bicubic sharper. Converted to Lab mode and unsharp mask on the lightness channel. If you skip the second step, it'll look more anti-aliased but I wanted to test the texture details here.