Faster than light travel is science fiction. So you could have that as an element in a steampunk setting where its possible yet has nothing to do with retrofuturism or steampunk technology as steampunk does not necessarily mean things that are scientifically impossible as we currently understand.
So many issues with that second sentence I don't know where to start. Steampunk was inspired by old science fiction and cyberpunk attitudes. Both parents are science fiction, as is the offspring. Science fiction does not mean that something is "scientifically impossible" or even improbable (not sure what you were trying to get at there). It is fiction based on or inspired by science, just as historical fiction is based on or inspired by history. Consider biopunk and related fiction. It usually deals with things that are already well within our abilities (or soon expected to be), focuses on aspects of that, then sometimes takes it to the extreme, or adds a plot relevant to that.
Steampunk is one of the most well known examples of retrofuturism. I don't know how you can think that you can include a steampunk setting and not have retrofuturism and steampunk technology. Steampunk, just like real, modern human society, is heavily influenced by the relevant past and present technology. The point of steampunk is the retrofuturism and the different path technological development could have taken, and how those would have created a different world.
In case you weren't aware, the *punk genres are often rebellious against, or at least shunning, the world that gave rise to them, and where it is going. Since I don't read the most recent steampunk I can't say if it is both the "real" world and the fictional world in that case, or just the "real" one.