I did a tour with a friend on his 800GS (17/21 rims), me on my 990 Adv (fitted with 17/19 rims).
On motorways we were same speed, cruising at 90 mph with ease. I had more top speed, but cruising was fine. His fuel economy was better than mine in terms of range and fuel used.
On smooth fireroad trails we were even. On washed out parts, rocky trails and grass I had to ride with more care than him. Some of this was due to my tyres, but a lot of it was due to the better agility and less weight of the 800.
Having said all this, he is not happy with his 800 and is looking for a 990 SMT........