It will fit but you will need to remove the top supporting bar, but on my standard GS all the adjusting mechanisms stayed in place.
When I had a GSA I could ride up to about 60 with my visor up. Any faster and the wind pressure forced it down (Shuberth C2). With the Givi AirFlow I've taken my standard GS into 3 figures (in Germany) with the visor staying up (same helmet). With more adjustments available I think it's better than the standard GSA screen at what it does but is a replacement GSA screen (possibly second hand) any cheaper and you would keep the bike 'standard'.
If you are thinking of buying new, GhostBikes on eBay are offering them for £140 with free postage, but if you ring them and quote the 5% discount code in Ride magazine or MCM (the free one), the price drops to £133.00
Mike