That whole weight difference between a foam hopper and a real hopper sounds like an April Fool's article to me. The foam hopper being light by 2 or 3 GRAINS.
Three grains is .007 of an ounce. If a person was able to make perfectly identical casts, time after time, the wind would make impose more than a 3 grain difference on the fly hitting the water.

If the argument was made that the 3 grains changed how deep the fly floated in the film, then the size (volume) of the fly would make a big enough difference to alter the buoyancy. Hence, making if float higher or lower in the water.