Will there be a significant difference ... or what is the difference between gettting 11 inch rears VS. 11.5 inch rears? is it just the looks .... does the wheel look sunk into the car or is there no diff. at all?

Dario

 

The difference in rim width will be small, but the look on the car will all depend on the offset.

 

As mentioned, position of the rim or tire wall relative to the fender is generally considered to be a function of offset (ET which means 'pushed in'). An 11.5J rim would have to have 5-6mm less offset in order to look exactly the same (fender line) as an 11J.
You are splitting hairs here.........but you may want to check the rim width is proper or ideal for the tire you want to run.