The driving line is a distraction and crutch. I normally drive from hood cam view, used to run bumper cam, but that gave me very little feel for the sides of the car and where they are in relation to the sides of the track. Lacking a triples set up and running in cockpit view, the only real option to allow you to get a feel for the car and where it is on track laterally, is third person view. I have never been able to use that view.
Since turning off the DL, almost 2 years ago, in GT5, and also turning off as many other HUD distractions, ie driver's list, map etc, I've gotten quite a bit faster. The first thing I did before turning the key on the first car in GT6 was turn all that off. The thing with the DL is, it's someone else's idea of a cautious and safe line and takes nothing of the actual driver and their controller setup into account. It certainly won't help you win many races. With the advent of new tracks I've never seen, I've put the map back on the hud until I learn the new layouts and remember what's coming, from one turn to the next. I've never used the gear flashy thing to gauge a brake point. It's on but I can't remember every paying attention to it.
Take all this with a grain of salt however. I can in no way be considered one of the fastest drivers in GT, but from my first online race to now, I can at least keep it on the track, most days.
Pick your own line and pick your own braking points, at least that way it'll be yours, even if they're wrong. You can bet the DL almost always is.