Cat-back means whatever exhaust that is dual starts at the cat. Dual starts at the manifolds and duals all the way back. You could have cats in duals if you wished and spent the big bucks.
What? cat back means that the parts you are replacing (with a "cat back" system) replace the factory equipment starting from (but not including) the cat continuing all the way back to the tailpipe. You can put a cat back system on anything whether it has duals or not.
I just put the corsa cat back on my 2006 overland. Sounds great when you get on it and there is no interior resonance at cruising speeds. It has a nice deep tone to it. I know someone with the Borla system and the Corsa sounds much better.