The transmissions on current GE and EMD diesel electric locomotives are about 94% efficient from the output of the prime mover to the driving wheels. I would expect electric car motors to be on the order of 90 to 95% efficient, so this should compare favorably with a mechanical tranny.
Speaking of locomotives, the free piston gasifier was being heavily researched in the 1950's as a more efficient realization of a gas turbine and something that could compete with diesel engines as prime movers.
One huge difference between attaching something to the phone network and direct back-feed connections to the grid is that the back feed can be deadly if there's an issue with the local distribution network. This is usually taken care of by the anti-islanding circuitry in an inverter designed for grid tie use, and the utility has both the right and duty to make sure that circuitry is present and working in any grid tied installation.