at the risk of posting this multiple times (I seem to be having troubles posting?) I'll try again, one last time!
Cinema's render engine will calculate data based on 32bit at render time. When the render is complete, the image is converted to the bit depth as selected in the render settings. See Render Settings->Save->Depth. What you describe seeing in the picture viewer sounds fairly normal. Just be mindful however, that image formats may also determine the final bit depth when they're saved to file (some formats won't save in 32bit for example).
Re HDRI images - 32bit renders are effectively the same thing. The issue regarding the sliders not providing more "detail" when you change exposure levels might be to do with your lighting, the bit depth of your render, or both. Take a look at the red-marked areas in the two attached images below, might help. The scene light's brightness is set to 1000%.