Now, to properly compare apples to apples, look at
this graph (on page 10) regarding the Prius motor's efficiency at various power outputs (you've probably already seen this since you mentioned it earlier). Below 5kW, it's at 25%. At 10kW, it's at 33%, and that's a realistic highway speed power output. If we can assume 33%*90% (drivetrain) we get 29.7% - with a 38% Stirling engine, if we get 90% from the rest of the system (generator, battery, controller, wiring) - and I've already posted links showing that's do-able - then if your motor exceeds 87% efficiency you come out ahead. At 5kW output you just need 67% motor efficiency to come out ahead. It's nearly a wash, plus there's lots of tricks you can pull with electric motors when you have a handful of them (vs just one engine).