For what its worth I was wondering the same thing a few weeks back so I did some research from racing schools. What I found was this. You should always either be on the gas or on the brakes. Any coasting is slowing your lap times.
Now that being said I think our problem with rc is how much brake is needed and where do you apply the brake and pick up the gas. Personally I get faster lap times by coasting because I tend to over break the corner thus slowing my lap times. If you have great control over your car then braking the turn is the fastest. For most of us coasting will be faster and more consistent.