Founder; I've long been a proponent of the system you describe ie a small turbine driving an electrical generator which in turn drives an electric motor in each wheel hub. By wiring the motors carefully, you could use them as supplementary brakes by turning into generators when you release the 'go fast' pedal, and you'll get 4 wheel drive without the problems of a mechanical transmission.
As to fuel efficiency; the reason they were fitted with heat exchangers was to enhance this. I don't know exact figures, but I believe it could be made as efficient as a petrol reciprocating engine of the same power, the bonus being by operating at constant max rpm, you could adjust the fuelling to such fineness that emissions would be far less than the petrol engine.
Obviously a pure thrust engine would be a problem with a road car; the one illustrated by trublu 351 must be for either drag racing or LSR attempts by the look of its brake 'chute.