Conversion of linear motion to rotational motion is the loss you're looking for. To your point about effectively gas turbine/electric drive trains from your OP

Conversion of linear motion to rotational motion is the loss you're looking for. To your point about effectively gas turbine/electric drive trains from your OP, the problem is efficency in burning the fuel. This comes from the lack of achievable compression ratio in a turbine system. 6:1 is good for a jet, 13:1 is good for a N/A piston engine (not sure what race engines run exactly). Higher compression ratio = better fuel efficency.