I suspect that every roaster will have their own definitions of these roast degrees to suit their flavor profile. You should ask your roaster to define these in objective terms, perhaps using the Agtron scale as an objective reference.
You could then identify the roast level of each to a high detail of accuracy, perhaps defining your French roast as an Agtron 38, Northern Italian roast as an Agtron 46 and Southern Italian roast as an Agtron 40 - or something like that - thus giving a common frame of reference for discussion.
Regardless, you should cup each one to determine the how that roast degree affects the origin or blend in question.
Changing the roast degree will affect the taste; how much, will depend on the unique coffee or blend of coffees used by your roaster.
The terms you have used "Italian Roast" and "French Roast" are too non-specific to define the taste of your coffee; such terms are generally used by marketers to conjure the romantic image of some preparation of their product, but in truth, tells us very little about the package contents.
There are many other variables that contribute to the ultimate "taste" of your coffee, including by not limited to: origin, grade and the roast profile used by your roaster to obtain the defined roast degree.
The question that you have asked "is there much of a difference in taste between x and y" cannot be answered without more information -- this is similar to asking "which car is faster, the blue one or the red" without first knowing the make, model and configuration.
Hi Yall, I have now come to the conclusion that I don't use any fancy roast terms. I have three basic roast levels: Nothern Italion, Central Italion, and Southern Italion......Dark and oily, dark with a spot of oil, and cinimon colored. Of cource I'll roast to whatever, but the basic def is the baest pace to start. IMHO