I tell all my customers that the Session Steel is one of the best deals out there for diatonic players. It is my harmonica of choice. However, many Seydel products are modular in design and that means that parts are interchangeable among the different models. In fact, the open style covers of the 1847 will fit perfectly on the Seydel Session Steel. Several years ago, Seydel marketed a diatonic that it called the Silver PLUS. The Silver PLUS was discontinued when the Session Steel came out. Currently, the Session Steel comes with full length covers.
However, at 16:23, you may still purchase the Session Steel with the 1847 covers and I call it the Session Steel PLUS.
I don't think there is a great advantage over the 1847 covers and there certainly isn't a difference in tone or volume. My personal preference for the 1847 covers is merely because they seem to fit into my harmonica rack a little easier. Some players prefer the 1847 covers because of the open backs.
The above pictures are side-by-side comparisons of a stock Session Steel and a Session Steel with 1847 covers (Session Steel PLUS).