I'm around 5'8. I tried both the GS and GSA (2014 models). On the GSA I needed the low seat and even then could barely touch the floor. As I found the standard BMW low seat to be very uncomfortable, I opted instead for a lowered GS with a standard seat which fits me about right. Sargeant seats are supposed to be very comfy but I didn't have one to test with so couldn't make a purchase decision based on that.
It's not as simple as inseam as I'm sure you know, the width of your hips, height of the soles on your boots, size of your glutes and probably many more things come into play.
Personally, I'd love a GSA but the GS just fitted me better in the end and for me that was more important. Theres a video on youtube of a tiny guy, around 5'1 riding a GSA iirc. It can be done, but I prefer to be able to at least get the balls of my feet down comfortable, to allow for dips in the road, odd cambers, soft surfaces and so on.
My last bike was an FZ1. It's seat height was listed as higher than my current GS, yet in practice, due to the width of the seat and angles, I could get more of my feet down on the FZ. So really there is no substitute to trying the bikes with the various configurations at a dealership, the measurements listed don't really mean much, it's so individual. Maybe someone will lend you a low Sargeant seat to try, that might help.
