Was wondering about this too. My surgery is optimally next July so I have quite some time, I might try to increase leg flexibility if there is any benefit to it. This really depends on the biological process tendons go through during lengthening, I have no clue about this honestly. Some doctors say it's recommended some say it's not necessary but idk. Since there is not really a big sample size of leg lengthening that can be matched (many different ages, methods performed, etc.) I doubt there are certain studies about LL outcome with and without pre surgery leg lengthening. I think the primarily things to look for are age and general health (if you are a smoker you can't do LL) as well as physio therapy. What I have read is the flexibility and outcome long term (if done by a good doctor) is determined by the amount of "effort" people put into recovery (physiotherapy). it seems similar to a stroke, where when you go on physio as early as possible, trying to regain lost function, the outcomes are most promising. One famous skiing professional I know of literally obliterated his whole leg and he was able to regain professional sports again (after years of recovery). kinda insane. really depends on treatment honestly. tldr, yea you can give it a try, won't harm for sure. id like to read some statistics on the topic or so.