Does applying negative wing mean you're running the minimum available down-force with the least drag, or is there something else going on?
The angle only describes the position. Since it is an inverted wing, there is downforce despite the higher forward edge. You can have a look here, where Alex described it in detail https://forum.sector3studios.com/index.php?threads/rear-wing-at-bmw-z4-gt3.14589/ Oli