I think you have missed the point. It's not higher top speed that we are worried about. It's lower high speed cruising revs. I'd rather not have my engine screaming bloody murder at me if I have to do 70 for a stretch.
Ok. You have three choices, a bigger drive sprocket, A smaller rear sprocket, or a larger tire.As was mentioned, the way these gearboxes are engineered, top gear will always be 1/1 regardless of how many lower gear ratios you have. It would take a total redesign to have overdrive gears in this box. It wouldn't even be this box. the reason for this design is that in top gear the gearbox isn't using any gears, so is more efficient.Bare