The only responsible answer anyone can give you is that THEY DON'T AND CANNOT KNOW IF LENGTHENING >=7.5 CM IS SAFE FOR A GIVEN INDIVIDUAL. There is quite a bit of evidence that doing a conservative amount of lengthening (under 6 cm) is something people generally recover from extremely well, but there is almost nothing on how people who did greater amounts (7.5 cm+) will recover. It varies a lot in individuals - some may be able to tolerate 8 cm as well as the average person tolerates 5 cm, and some will sustain permanent damage by doing 8 cm. The thing is you have no way to know where on this continuum you will fall, and that's why I recommend not having a fixed number to lengthen in your head when you go into the procedure.
When you are faced with unknowns concerning your health and mobility, the only responsible thing in my mind to do is to err on the side of caution.