I had a top box on my ybr .. was fine. Big enough to hold helmet and a couple of other bits and pieces. never seemed to make a difference. And I didn't have to buy luggage rack as it already had one .I use a tailpack on my sfv now though cause I haven't got luggage rack, both work ok for me ... I think it's down to use and personal preference really
only got little tailpack now.. but big enough for carring what I need and I have got larger Oxford Soft luggage if I need more room ...