I've never played with anyone using better than Inhuman Speed. I'm not sure I would allow for that kind of hit-and-run style, but I *might* allow more of a jousting movement, where you move into a zone, attack, and continue on to the next zone beyond it. The idea there is momentum. There are examples of it being done in the novels, IIRC.
Depending on how much this messes with standard combat, though, I might make it require a stunt. Also, I would certainly allow targets to declare on their turn that they are ready to strike when the speedster enters their zone. I can't in good conscience allow this kind of thing to make a character effectively immune to melee.