actually higher FD is for high revving cars.
a 4.785 on a stock Y21 with 8K rpm limit
1st gear (less than 60kmh)
2nd gear (less than 100kmh)
3rd gear (less than 140kmh
4th gear (less than 180kmh)
Not very nice numbers, isn't it?
As for the highway issue, it's not really a problem with the Final Drive, but more with the ratio of the individual gears, specifically the fifth gear.
If anyone has looked at the ratios from KAAZ, ATS, OS, mFactory etc. etc., you'd have notice their ratios are different. Some will run well with 4.2 FD, while some will need a 4.7-5.0FD. All depending on the individual gear ratio.
The gear ratio is used to depending on the torque band of the engine.
If you have a wide torque band engine, you can have RPM drop from 9K to 5K, so you can use a wider gearset (more speed gain before you shift)
If you have a narrow band torque engine, like the B16A/B, rev drops will usually want to be minimized, like maybe from 9K to 7-7.5K, since any less would be outside the peak torque range.