There really isn't anything in GTA that demands super high bandwidth I/O, unless they plan to make a hypersonic car/aircraft.
Character switching isn't always instantaneous in V. Only when characters are very close, or in scripted mission scenarios.
Planes are also much slower in the game than the real thing. Partly for gameplay reasons, very likely, but probably also because of streaming limitations.
Better graphics also means bigger assets, requiring better streaming.
And something that has been asked for since ever for GTA games: more building interiors. That's the one that's likely limited by streaming speed the most. Interiors require A LOT more asset variety and density per unit of area. With SSD, it may become more feasible, though.