I have the 58L Givi box on my GSA, combined with the BMW alu-cases. The Givi case needs an extra plate mounted on top of the rack. Fitting takes 5 min, so quite easy to fit. I use the BMW 35L topbox most of the time, riding solo, but when I bring the boss along the extra capacity is appreciated. As a bonus the Givi case is also positioned 5 cm further back allowing for more room for the passenger. I have also used the 58L Givi riding solo, combined with the BMW side cases. Up to 185 km/h the ride is fine, but at 190 the bike starts a small weave. So for my riding I consider the speed instability a none issue (the label says max. 120 km/h)
Also, the 58L Givi blocks for the lids on the side cases to swing open, but by opening both latches there is plenty of room to lift the entire lid off. When I ride with the boss we usually use inner bags, load them in the morning and we don't access the sidecases during the day. We only need to open the top case, thus the slight blocking of the lids is a non-issue.
All in all I can recommend the Givi 58L if you need the extra space.
