"Unpremultiplied Alpha" - That's not a Corona/3ds Max feature, but the 2D editor's, right? You can render an image with an HDRI background, or anything else, without using the black background solution, load it in a compositing software, and specify premultiplied/unpremultiplied alpha there. There were many forum threads in the past where we shared how exactly the same data loaded into Photoshop looks "wrong" compared to Fusion or Nuke.
"Scene rendering longer on a black background" - The link points to a thread where the issue was related to using the noise limit. Can you confirm that everything is working as expected with pass or time limit? That is, the render time is higher when using noise limit only, and when using pass limit or time limit, there is no performance drop when using a black background.