Difficult difficult to say yes or no to become a WHS. We already have the Medina of Tunis and Sousse which I also visited. They are touristy. Sfax is authentic. I was the only western guy there as I didn't see any other tourist the 4 hours I walked through the Medina. 
In Sfax you can really see the local life and how it all goes. I love that. The narrow streets, the people and the smell. There are a few old houses open for museum and looking at the guest books they don't get many visitors. Dar Jallouli is the most famous. These old houses have beautiful courtyards. Of course there is a kashbah as well. Old mosques and the wall which is still very complete from what I could see.

So WHS or NOT?
Maybe we can exchange the Tunis Medina for Sfax Medina :-)
In anycase if you visit El Jem (from Sousse) which is half a day trip, continue to Sfax for the rest of the day. It's amazing to wander around.

(photo copyright CTB Global - permission to use on worldheritagesite.org)