All the pro FPV builds and the more expensive quads don't seem to be using plastic props. Any reason for this?
Plastic and wood props wear and tear over the time while carbon fiber props have very less tendency to wear and tear hence are more durable ad last long. I have seen plastic props breaking in flight as it was used for long duration. So if you are making expensive setup the it is always better to use carbon fiber props as their strength is far better than plastic or wooden props.
Also carbon fiber props are more rigid than plastic props. Hence you can have better efficiency out of carbon fiber props which will result in less power consumption resulting in more flight time which is a very crucial factor while designing and build qauds or any multirotor for FPV.
Maybe this would help. The flex is just a twist. The twist causes a lower angle of attack, progressively worsening the further out the blade you travel and as the revolutions are increased (where and when you're getting your most lift). It's the point hitting the air hardest.
Less angle of attack means less bite, resulting in less lift and less response (this is also the reason variable pitch works). The denser the air, the less angle of attack needed and vice versa. Also the twisted blade takes less power to rotate. Flatter blades give less lift and less response but will increase the flight times due to less drag on the twisted prop. A prop is just a wing that rotates - drag and lift.
The tests were done long before quads through. Sikorsky might have been the first to do wind tunnel and lift capabilities tests I think if you would like another place to search. I hope this helps.