Here's a photo of my GSA with Givi panniers and top box. The top box is the Maxxia 52, I think the panniers are called E360's. They're pretty capacious, more so I think than the BWM system cases, and the boxes themselves are of an ok quality. The panniers have been all round Europe with me on this and other bikes without ever leaking or giving any problems.
(Edit: Just re-read your post and see that you already have the panniers, d'oh, but I hope the bit below and the photos are useful)
I think that the mounting hardware is of more marginal quality - I had to bend the pannier frame a bit to get it to line up with the mount points and after a year and a half it's showing a few points of rust. I've also had to wedge some rubber sheet under the topbox as the locking mechanism doesn't hold it snugly enough, so it can rattle a bit going over bumps. With the panniers mounted the bike looks a bit fat-arsed from behind, but they're no wider than the handlebars.
I'm sure that there are sleeker or more robust options available, but the Givi stuff gets the job done and isn't too ruinously expensive.