Durability was a bad point in the Guru Reports for Souveran. Much less than the carnauba winner, P21s/s100. In their shine tests, P21s also beat Souveran in most of the categories.
Going back to the original question, I would strongly recommend against topping FMJ. FMJ looks awesome all by itself. However, if you really want to use a carnauba topper, I'd give my vote to S100 :xyxthumbs
I really like Meguiar's deep Crystal Step 3 Carnauba wax. Locally available, inexpensive (under 10 bucks last I checked), easy to use and looks great with good durability.