An XL Schuberth C2 will not fit in any BMW case for the GS, at least I have not been able to make it fit. As to when I use a case, the top case is mounted most of the time, side rarely. I ride to work and need to carry my work clothes in the case, get off bike, would like to leave helmet with it instead of carrying into the office, but it wont fit. As for locking it to the bike, cant do it with the Schuberth, no D ring and I wont cable lock it to the bike and leave it unattended in a parking garage.