I am a true, original Italian Iacobaci. I live in Sicily, and I may tell you the correct original pronounciation of our name: "yakobatchee".
In fact, I've chosen this name for my e-mail address so that anyone from any country (I'm a translator, I need to keep in touch with a lot of foreigners) may immediately know how to pronounce my name!
At the times of your ancestors, nobody didn't probably give a damn to the fact of losing the correct written form (which maybe they didn't even know!), simply because in that period nobody could read. Or maybe, the officers of "birth offices" simply used to write down what they had heard, and that's why Iacobaci and Iacobacci (the original names) changed so much with the passing of time!
Keep in touch,
Giuseppe Iacobaci