I have the Givi 58l on mine DL650A and is fantastic.
It's a big case, but when mounted on the bike it doesn't seam that big, and you can put 2 helmets no problem at all, and a few more things.
When windy you will feel it, the bike becomes more unstable. On the road, don't feel that much, maybe a bit drag but nothing that makes me sorry for buying it.
I got the 58L because I need for my job, and I like to have all that space :P