I was doing some Q laps at Le Mans this morning, and got bored, so I decided to just screw around and see where you could break the rules and get away with it.
Firstly, I wanted to see if they fixed an old exploit that I remembered from GT Sport.
They haven't.

Basically, it allows you to bypass the final chicane before the start/finish line. There's a method I discovered that allows you to go full-throttle through there without a penalty. You could probably get about a .3-.4 advantage (Qualifying only. It wouldn't work in a race).
I didn't/wouldn't use it to set a lap time, and don't recommend it, so I won't explain how it works.
Also, somehow the game let me massively cut the Mulsanne corner. I drove right over the green bollards without a penalty.
I used to do this sort of thing all the time in GTS, just to find out what the developers overlooked. I haven't done it in GT7, because I figured that everything got fixed. A lot of it has, but apparently there are still some things they've missed. I'll have to look around some more.