The chance of the instant kill to follow through is 1.98% of the time or 2 out of 101 tries according to the UESP. It's entirely possible, but the probability is extremely low and shouldn't be taken for granted. If you wish to instant-kill Morokei, you're better off with the assassin's blade/shrouded gloves/dual-wield daggers combination if you're invisible and have a relatively high sneak level.