I think proportions would be perfectly ok. It's a wise, saffer choice.
Personally, I would also consider maybe "only" 7 or 8 cm on tibias (NOT femurs despite general impression). depending on clinical and your individual proportions evaluation, to be even safer, simple and cheaper. But in this case with externals more or less classical llizarov and a proven experienced doctor. If you have to stop before 7 cm or if it's not enough for you then would do also femurs.
Remember allways to balance height gain, safety and avoiding loss of functionality.
