Yeah the back box does have a lot to do with dampening the sound down but so does the middle box (resonator). I run this setup below for 3 months and it did make the exhaust note totally different. The setup was cat + mid-box + no backbox. I switched the milltek pictured above, so no cat, no midbox but a large backbox and it is much quieter.
Sorry for all the picture hore-ing.