The difference in the resonated exhaust is the centre box. The non resonated version just comes with a pipe, and the two are interchangable and can be bought separately.
The back boxes are overkill though, I ended up have a back box delete pipe made, but if you do that, you'll definitely need the centre section as it is very loud without!!
Other option is to have a smaller back box made, but for the price paid for the milltek, you may as well have had a custom system made, like BCS for example.
Jas