I had one, but just sold it, not because of the carbon, but because it was a little longer than I wanted. It was 68 inches. I've determined that 66 inches is a better length in that design for me.
Anyway, to answer your question. It's difficult to say. Craig at HH Archery says carbon adds about 5-8 fps. For an accurate comparison, one would need bows of the same weight and length, one with and one without carbon, shooting the same weight and material arrows.
My carbon backed Wesley Special was 59#. The closest Hill I have to it is a 66-inch, 55# Big 5. No chronograph, but the heavier bow felt like it shot faster, but no idea how much is attributable to the carbon vs the heavier weight. But it didn't seem like the difference was that great. The carbon backed certainly didn't blow away the glass backed bow. That's why my new bow on order will have glass on the back and belly. Sorry I can't be of more help.