The backing, glued in and fitted tight, will let you use a larger rountover bit without losing strength. When I look at PA and loarge studio speaker cabinets, I notice that they have large roundovers, however, most are made of MDF. It is very hard to get a grip on such an edge, given how heavy speakers and ply are. So I hope you're working on some sort of grip, possibly a recessed grip. I do AV for our senior group and hate wrestling with large, gripless speakers. Not sure there's much advangage to using box joints if you're going to do roundovers, and even less advantage if you're going to cover the outside of the box with some sort of fabric.
The more I do, the less I accomplish.