Is this one surgery on the femurs? If so, it's not really matter of whether you want 10 cm or not. At that length, it would be more about how much your body can handle. Once you go past 6 cm on femurs, you entering a risky area where your soft tissue gets very tight from being overstretched. Past 8 cm, and you risk losing your athleticism significantly. I would stick with 8 cm, as I don't think the final 2 cm is worth is the risk and loss in athleticism. Of course that changes if you find out that your body is still growing and can adapt to the changes. At that point, you'd have to find out from your doctor if you can keep going.
If you mean 2 surgeries, then 10 cm is probably more worth your time/money for putting yourself through so much. You probably will not risk much athleticism with 2 surgeries.
Proportion-wise, I think 8.5 cm difference between arm length and height is quite significant. It'll probably look like you're wearing high heels. But I don't think it's significant enough for people to think you look silly.