"too much backpressure"

When backpressure becomes "too much" the engine will not be able to reach peak rpm. It will lose power. It's very simple to judge when the backpressure is too much.

Deflector's don't always produce too much. Short ones usually don't show any effects at all, other than keeping your model clean.

It takes two flights to test. It takes weeks to argue and where are you after all the time reading. Our hobby is kewl that we can find our own answers in about 10 minutes flying.