According to ASP-Fighters, another reason for looking at the high-wing F-5 was to remove the requirement for the wing to be thick enough to accomodate the u/c legs. Now as has been pointed out, standard F-5 wings do hold the u/c legs and are pretty thin anyway, but if you wanted to make the legs thicker and/or longer, say for rough-field capability, then it would become an issue.
You could say that, in essence, a high-wing F-5 is approaching the condition of a Jaguar, which is exactly what my FA-5K was intended to be.