cv, that would be what's known as a tailbag. There's lots of good ones out there (check out the Cortech ones), but IMO, none compare to the Givi topcases. And to answer your question about the weight distribution, if anything, my topcase seems to make my FZ smoother. I don't know if it's just that I've found a good weight distribution factor or what, but it definitely rides smoother with the topcase on. Oh, and I've never had the front wheel off the ground and routinely ride with 12-15 lbs in the topcase (Givi recommends a max load of 22 lbs, at least for the V46).
And ant is correct...the topcases, at least the Monokey ones, don't come with any sort of mounting hardware. You'd have to purchase the FZ351 topcase rack in order to mount the case. I'm pretty sure (though not positive) that they don't make the Wingrack for the FZ6, at least not anymore.
reiobard, it's simple to put the topcase on and take it off the bike. The plate has two mounting points that slide into two receiving points on the case, turn the key, push a button and snap, it's locked on. The rack doesn't come off and on, although you could do that pretty easily if you wanted. But I think the rack, plate, whatever you want to call it flows pretty well with the lines of the FZ, so I've never bothered to take mine off and on.
Here's a picture of my setup:
