I dunno Erik, but it seems to me that it would be pretty hard to overload an OEM top box considering how small the inside volume actually is. OTOH. I do overload my Givi V46 when on 2-up tours, to the point where it is often hard to close the lid successfully, and that box is an honest to god 46 liters inside. Having the steel rack supporting it gives me a little more confidence that I won't crack the subframe of the bike. But I do know I'm playing with fire there.
As for use as a back rest, that is the
only time I will use mine (when 2-up). Otherwise I have no problem fitting whatever I need into the side cases. Maybe if I was going on a week long vacation one-up I'd still use the trunk. Not sure because the long rides we always do together.
Like the OP I hate the looks of any top box, but when riding with the missus she insists on having a backrest as it makes her feel much more secure. And I find the Givi V46 with the color matched lid to be the least offensive of the bunch. Here's a picture of the bike on tour in Cape Bretton, Nova Scotia last year (the Bagster tank cover and tank bag have since been replaced with a different setup)
A Corbin smuggler is slick, but very expensive and doesn't really store much. You could put more into any decent size tank bag and it won't cost nearly as much.
Obviously, as with anything as subjective as this, YMMV.