Gene therapies are pretty incredible. Some of them are still making a button-hole with a machete, but that's relative to the previous medical intervention of a button-hole with a tank's main gun.
One of the treatments for sickle-cell involves switching off the gene that makes the malfunctioning red blood cells, but of course that's not sufficient; you'd stop making red blood cells completely and you'd die. So it's combined with a modification that switches on a gene that all humans express pre-birth that causes your body to make "super-blood": red blood cells with significantly more binding points for oxygen. This is necessary because a fetus gets oxygen from its mother's blood, so the increased binding affinity is useful for pulling the oxygen towards the fetus at the placental interface. After birth, expression of that gene is disabled and regular RBC genes switch on.
So the therapy doesn't "fix" sickle RBCs; it disables the body's ability to make them and re-enables fetal RBCs! I have seen no literature on whether having fetal RBCs in adulthood has any benefits or drawbacks (besides changing the affinity ratio for their fetus if the patient gets pregnant, I imagine increased-affinity RBC could help for athletics... But I also imagine it requires more iron to generate them so has dietary impact).