I've always thought that "plethora" was closer to "a large number" while "surfeit" denoted a clear excess of something. Indeed, the google defines plethora as "a large or excess amount" while surfeit is "an excessive amount of something".
So, is the general consensus that a surfeit is more than a plethora or is that just my distinction?
I've recently started getting quizzical looks when I use the phrase "we have a plethora --- one might even say a surfeit --- of X". But maybe the looks are because nobody really talks like that, rather than my uncommon understanding of the meaning of those words!