The license plate thing I can confirm but not for use with radar but with lazer. You are trained to first target the license plate and later the tail lenses.
The license plate covers are suppose to cause it to not reflect the lazer as much making it hard to get an accurate reading. Therefore the cop has to do it again trying on your tail lenses. By then you shoudl have hopefully slowed down. Assuming you saw him/her.
Personally I wouldn't waste my money on those products get a radar detector and drive safely. Not just so that yourself is safe but so that you don't cause problems with the other idiots on the road.
Note: I never said you couldn't speed lol.