Don't do it.
I did exactly what you are thinking of doing. Went from a 1200 to an 800GS. Just under one year later, now happily back on a 1200.
The 800 was very nippy around town, but as Bryn says, there's not a lot of room when two up. The bike would happily cruise at 80+, but felt very flimsy compared to the 1200, which feels much more substantial. The 800 was very vibey and buzzy at speed, whereas the 1200 just purrs along.
The clutch went on mine and was replaced under warranty. However, it was the chain snapping during a fast overtake that made my mind up that it really wasn't the bike for me.
If you're into off roading, then yes the 800 feels much lighter to manouvre around. If you want somethiong with plenty of grunt and room for two plus luggage, you may want to think again...
They are both good bikes but for different purposes.