I've already played some games in Sicily, and definitely I dont support at all the idea of becoming vassal of the HRE. Anyway, to achieve good things is not as hard as some people say.
In my most played game, I started as Count of Napoli (changing his culture to Egyptian in the savegame. Just for rolegame), and instead became a vassal of the HRE. When I could hire mercs enough, I conquered Sicily withouth problems, and waited until it started to give me a good income. Later, I got money, money, more, and MORE money until I could declare war on Egypt, and took Alexandry. Later I requested the Pope to invade Apulia, and became soon the king of Sicily. Later I became also king of Egypt.
The problem? The kaiser took himself the liberty of helping me. And by helping me, I mean that he started to conquer a lot of territories which I wanted sooner or later, and which I couldnt get because of the crown power (no wars between vassals). Here was when I started to make reloads, reloads, and MORE RELOADS. It is fucking hard to become independent of the HRE, I couldnt even do it when the whole Italy, Bohemy and some German duchy tried to do the same. I abandoned the game and started a new one.
The HRE when I left:
...And my territory:
My conclusion: to become a vassal of the HRE is an option, but not the best one. As far as I played, it's possible to defeat those annoying africans as Apulia by hiring mercs and good management.