How important is relativity when applying it to fast than light travel?
Your question should be reversed. The correct question is: How important is the speed of light when applying it to relativity? You see according to relativity as you approach the speed of light time changes, but you can’t exceed the speed of light. There is no faster than light travel.
I addressed a similar question in May when someone askedI replied you’d be the same age, but people on Earth would be much older when you got back.