Karlos,
I have owned the factory low chassis GSA-LC and currently own the standard GS and they feel/felt the same height to ride.
gspod,
You can buy a lovely GS/GSA Exclusive seat in three heights, low, standard and high.
This is the part number for the Exclusive High seat...77 34 8 553 995
To quote what it says in the BMW mag in front of me, the high seat gives seat heights of 870 and 890.
I think that should sort you out but they are expensive, however I seem to think that you can also purchase the latest ‘comfort’ seat in a high seat version, but check with your dealer to be sure.