I have a Givi Wingrack 2 with a 52ltr topbox and 2x 36ltr panniers. Have had a better experience with them than Kebab though as he said there isn't a lot of room for the pillion's feet. Having said that, no trouble getting on or off and and I'm glad to have it and won't be selling it until I sell my bike. We've done several long rides and it didn't cause any issues. Perhaps there is a smidge more room with a Wingrack 2 compared to the 1 (which I think Kebab had?).
Don't know, but if you want a 3 case hard luggage option, the Givi system is probably the easiest to get hold of. Wingracks are easy to come by although you will need a FZS600 specific fitting kit which is not as easy to find. Kappa make identical kits so you can expand your search for them too.
The Givi system is probably the best for the FZS600 although if you are doing regular trips 2-up there are probably other bikes that lend themselves to the job more easily and ultimately do the job better.
If you go with a FZS600, the easiest solution is probably a large topbox, tankbag and soft panniers.