Most people use turbo because in some races, the race rule will only allow you to use a set motor winding. (ie 11.5R or stock spec) In order to get more speed from this motor, you can add turbo/boost to make it faster.
With boost/turbo, the motor and esc will be hotter. so it may decrease its life span or even burnt out.
So for fun, its better to run no-boost/turbo and just use a faster motor.
It is hard to say. But with boost/turbo ON, ESC, motor and battery are all hotter, so it is less efficient. So I would say boost/turbo uses more power.
Hara used a 17.5 in a TC race in the States last week. His 17.5+boost+turbo time is only 0.3s slower than his own time in Mod TC. The circuit is small and technical.