In most if not all the GV, the commander and other crew members were able to stand up with as minimum as half of his body outside the vehicle. From there, they had a perfect visibility outside the GV and they usually were equiped with binoculars.
IMO, external views are correctly dissabled for GVs, but two more views should be implemented:
1 - standing up with the head out and well above the top of the GV.
2 - Same as previous one, but with binoculars (same effect as tank gun sights).
In the two cases the head movement should be as free and quick as in airplanes.
In both cases you should be able to drive, accelerate and brake your GV, perhaps even to control its weapons.