Hi, as far as I remember, you were considering surgery using the precision method. You will most likely succeed in the 8cm femur lengthening. Like you, I'm also planning to lengthen my femur by about 7-8 cm in the future. What worries me most isn't so much achieving it, but rather ensuring my wingspan matches my height I'm probably going to be 173 cm tall with a wingspan of 166 cm if I grow another 7 cm, and that's what's bothering me. . Other than that, I think you shouldn't worry too much about success; you'll manage it.

edit: I saw your message asking for advice on preparation after I sent it. For bone health, ensure you have the correct amounts of things nutrients like vitamin D and calcium before surgery. The more overweight you are, the more weight the Precision device will have to carry; perhaps you could try losing weight if you have excess weight.
While it's true that building muscle increases pain, building muscle in your legs—not like a bodybuilder's—will help the mudscle adapt more your new bone easily, and your post-surgery walking will likely improve faster. Flexibility is key; I've heard that competent doctors wouldn't even operate on a patient if they found their flexibility insufficient an someone . You can improve your muscle flexibility through places like Pilates or physical therapy.