@Yourself You realize that some of the species they chose to "mix" in with, in the manga,have objectively inferior organs, right? Trust me, you do not want an insect's "eyeballs",
or any organs lower down the evolution chain for that matter. If the goal was indeed physical fitness, then gene engineering towards myostatin definiciency would have been more than enough (it's something we can do at this very moment, and which we sometimes employ on animals).
The premise and resolution is simply bs, the manga was enjoyable though.