After also changing from an 800GS to a 660Z the beginning of the year I can just say that the Ten creeps under your skin. Every single time I have to go somewhere on the bike I get excited. Mine has enough power and the handling is much nicer and more forgiving than the 800. I've done two off road trips with it and was saved a few times by the 660 where I would have definitely gone down with the 800.
For up to 150kph the 660 is fine. My 800 got a head shake at about 140 so I couldn't cruise any faster in any case.
KTM 690 R in another class. Really an enduro bike made into an all rounder. But as a true all rounder few bikes can beat the 660 Tenere. Just my humble opinion.