FWIW, I'd go for the narrowest possible - it only needs to be too wide once to be a real pain (perhaps literally).
The boxes on my R80G/S (homebuilt) -are 12" / 30cm
narrower than the Touratech medium and wide combination on my R1200GSA. That's a huge difference.
IF you choose another option than the OEM, a narrower box on the pipe side should be able to balance it out adequately, just try for narrower than the bars.
I've got black racks on both, and they aren't so visually obtrusive they bother me. Chrome, yes, ugly. Note that I'm not too fussy about looks, function first!
Second item - we're no longer fans of top boxes, in fact Susan won't ride with one anymore, and much prefers a roll bag. MUCH safer in the event of a rear-ender, or any crash, and more comfy for her to lean against. And it's a lot more flexible when it comes to load it up, and I can easily carry it over a shoulder with a saddlebag in each hand if need be.
If your reason for a top box is you want to put your laptop / cameras in the top box, then I recommend the saddlebag or a small tank bag. The top box gets the MOST shaking / shock of anywhere on the bike, not ideal for either piece of equipment.