In theory, you can go up to 12.2 cms with Betz Bone, but I would not even dream of going that far... After 8 cms, the recovery can be very very long. As you point out, you are not only lengthening the bone, but also "soft tissue": nerves, muscle, tendons, etc. You can distract at 1mm a day according to the Ilizarov principle, which is the one virtually all LL doctors abide to:
http://en.wikipedia.org/wiki/Distraction_osteogenesis
http://en.wikipedia....Gavril_Ilizarov
I know 1.70 m is not that short, but I wore lifts for a while and I can tell you: 7 centimeters will allow you to see the nicer side of society's face ;-)....I know it sucks, but hey, who made the rules!? :-D
By the way, how tall are you?