So uh how ethical would it be to hypothetically know all that but do it anyway for the knowledge it will provide us for the next iteration.
Many people are interested in seeing CRISPR push forward, and mention something like you have: but what about the "knowledge it will provide us for the next iteration". Trust me, I'm just as eager as anyone. However, if this mavrick experiment fails, and something is remotely off about these little girls (u can bet they will be under a constant magnifying glass), this will end up being a huge setback. It will spook the public, and the legislation factory will churn out regulatory policy that nobody wants.
It's dangerous to tinker without a truly full knowledge of what one is doing.
CRISPR Twins Aborted
...it's suddenly a political issue. An issue that could result in ~half the US population calling for regulation due to safety concerns and the other half demanding regulation from an ethical pretext (so, religious reasons).