sounds like your cat is stuffed, but i am not sure about the oil.. sure overfilling will make you burn oil, but i dunno about a plume of smoke..overfilling oil can make you blow all kinds of seals tho
what your cat goes and the internals disintergrate, you can have intermittent power problems, i know when mine went i was on the freeway and it just woulndt rev, had my foot flat and it barely made it to 100kph...pulled off the freeway think i had a major problem....but then as i was pulling off it went back to normal and was fine for a few days...then massive loss of power for a day...then fine..then screwed etc.. till finally it stayed screwed and then i got a new cat :P not expensive, should be less than $200 including fitting etc.
(another thing is that if your cat gets full of oil...then you go for a long drive the oil heats up and accelerates killing the cat...but a cat full of oil, when it heated up, would prolly cause a nice big plume of smoke too )