The Insight's engine is an incredibly efficient one, and it mostly operates in the 30-40% efficiency range. The Insight's electric motor + inverter operates around 85% efficiency, but after losses in your charger and battery, figure maybe 75%.
100 highway mpg does not seem like a stretch to me for an ecomodded Insight driven at normal speeds. However, I've done the math on further gains, and I can confidently say no car with 20ft² of frontal area will achieve 200mpg at 55mph any time soon.
I feel you could "easily" exceed 200mpg in a streamlined feet-forwards motorcycle or trike with an Insight engine. Apparently, you can do it even with a Honda motorcycle engine:
214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com