Same here 6ft, history of back problems and achey joints. Had a go on a mates Tuono and loved it. If you love sports bikes it's a great compromise as it's just a naked Mille with bar risers.
Surprised you find the fazer cramped. How about some rear sets? Also i can't recommend stretching enough. Hamstrings, Glutes and lower back. I'm sure Mortischa could advise you better than me.