This is the thread I was referring to. The consensus there seemed to be that even a nondestructive, therapeutic use of genetic engineering would be immoral.
But it is not the therapeutic aspect that would be immoral. The context on that thread was "designer babies", and you yourself seem to suggest it by making the contact lens analogy.
Beside destruction of embryo, even in therapeutic use we should distinguish between a health defect and cosmetic improvements. Further, the Church doesn't like any usurpation of the power of God as creator of life.