5 or 6 speed ?

If the idea is to get your highway rpm down that can be accomplished with numerically lower gears in the diff. Nothing magic about overdrive. It's really just math and the overall final drive ratio (including tire diameter) will determine rpm at a given speed.

Now that I've stated the obvious, the advantage to 5 or 6 speeds is that you can keep your engine in the optimum power band. Big torquey engine in a light car may not need it. Still would be nice. Could be expensive.