The graph I have seen on the forum says there is a slight depth increase, depending on the object. I haven't heard of anyone using it yet. The 8.5x11DD seems to work so well not many people appear interested in the larger coil.
If you are looking at coils, you would be better spending your money on a smaller coil. The 5x8 works great, and I get very close to metal fences, and trees, etc. And it's lighter and creates much less drag in water.