I think I recall seeing the mod you are talking about, regarding a mob's "con", but I don't remember off the top of my head which one it was either. I'll poke around tomorrow and see if I can find it for you, if no one else has remembered in the meantime.
Regarding the "how much exp I will get from killing mob x" I am not sure that such an addon is possible, or if it is, that it has been created yet. If anyone can answer better than I, please feel free, as I'd like a definitive answer too.