Not sure on that smell, if it goes away then I would assume it's just the desulphation process. On the other hand: "As the cat fails, it stops changing hydrogen sulfide into its odorless counterpart and the result is a strong smell of rotten eggs from the exhaust." Or have a look here:
https://www.e90post.com/forums/showthread.php?t=1369192
Out of curiosity - what codes do you have?
My 24 MPG seems to be messed up badly comparing to others, I really hope 30E9 code is the reason and NOXEM will fix that. If not, I will be clueless...