Rstudio has a great code completion feature. It provides a quick view of functions that start with a given string, as well as function and parameter definitions.
ESS is powerful enough, familiar to me, and integrated into Emacs, where I conduct most of my work – so I am hesitant to move, but this feature is making me consider such a move.
Is it possible to integrate this feature into Emacs ESS?
Is there anything similar to this for Emacs ESS?
Any hope that there will be (and if so, how could I support such an effort?)

You do get the completion thanks to the rcompgen package by Deepayan (now “promoted” into base R as part of the utils package). So when I type
and hit TAB a new buffer opens which gets me the left-hand side of your window above: the available options to the function at hand. I don’t think you can show the help directly though.
There is / was also a way to get context-sensitive help in the mini-buffer when typing but I have forgottten how/where that gets turned on.