And you'll find the Volt outperforms the i3 Rex in terms of hybrid efficiency and performance. When the i3 is in range extended mode it's pokey; and only meant to go a short distance before one gets a charge.

Is that a limitation of series hybrids, or is that a consequence of the engine used in these specific models? The gasoline engine in the 2nd generation volt has 3x the power of the one in the i3 (75kW vs 25kW).

Probably both? I do know that GM ended up going more "parallel" with Gen 2 than with Gen 1. And Gen 2 has more torque than Gen 1, despite switching to a lower displacement engine and going to an Atkinson (tuned for electricity generation) cycle.