Challenge excepted haha.
This is a interesting question because "sleeper" implies that the guy was powerful in someway (connections, earning ability etc) as well as being under the radar. Alot of the members of this Family was under the radar but not with the credentials of a sleeper. I am sure there is at least one name that we've seen that was a "sleeper" and possibly someone who was never mentioned in any records because this Family was so tight-knit.Now I'm speaking before the 1980s.
A few possibles come to mind. And they are the guys that I am the most curious about and couldn't pay someone for information on them

Michael "Mike Angelo" Angelina- He was on the Valachi 1963 chart. I have found any fbi documents on him but if Valachi could Identify him, he must've been someone to know.

Frank Labruzzo- a Caporegime, relative by marriage to Joe Bonanno. We know he had some loansharking going on and was in the garmets but nothing specific. Which I find interesting, seeing how close he was to Joe and Bill Bonanno. (Hardly even mentioned in their memoirs)

Pietro "Skinny Pete" Crociata- old timer apart of the Sciacca-Rastelli regime.

Michael "big Mike" Allegato - also listed on Valachi's charts, apparently was associated with the Asaros

After the 1980, I don't know.