This is a great idea though I am not sure if I have seen a similar suggestion before. Anyway I would love to have those stats in game so you can actually see if your guildmates are right when they say things like "Pff, did your dungeon 1st try."
Also as you said this woud be a good indicator to measure your dungeon's weakness or strength. But in my opinion just a table below the "Test dungeon" button in the guild chat containing the amount of deaths of the players who actually managed to complete the dungeon would be enough. I like keeping things as simple as possible.