I did once fit an Ohlins to my 1100GS (the BMW shock damping screw broke) and didn't really notice the big difference some folks seem to talk about, 'though I did notice a wider range of adjustment.
I've fitted Öhlins to my 2006 1200GS, and the really big difference for me, is when I drive standing up.
My bike is much more "adhesive" to the ground than before with the original ~70 000 km ones.