@james the issue in a series hybrid system isn't so much maintaining a speed, which requires relatively little power, but rather acceleration after the battery is "depleted". A Volt genset, if it had to handle the task alone, would have a very hard time passing someone on the highway. This is why they only have 8kWh usable but the overall pack is 16kWh. It gives them a lot of cushion to allow for the battery to add energy for acceleration even after the 8kWh is done.
There could be cicumstances where someone is doing a lot of hard accelerations and passing or maintaining high speeds for long distances where the pack will truly be depleted and the car's performance is significantly reduced.
I consider this a point of advantage for EV over series hybrid, but GM has designed around it so it isn't going to be a common problem. I don't know if fisker is doing something similar. They have spec'd 50 miles range with 20kWh which seems to assume the same Wh/Mile if they are also only using 10kWh as the usable range. Since Wh/Mile is likely to be higher than the volt, they will likely need to use more of the battery to achieve 50 mile range.
But the again they spec'd a very powerful ICE for the Genset so I imagine they can do more under genset power. Downside is poor efficiency.