Here the missing piece of the puzzle:

gravity is the single biggest factor in causeing your arrow to drop, the further it travels HORIZONTALLY the more time gravity has to pull it back down to earth. The vertical distance (the extra 6.6' in your 30' scenario) doesn't matter because its vertical, ie. dropping already. Gravity effects the horizontal distance only, so these devices eliminate the vertical portion of your arrows flight for you.

Not trying to say it isn't gimmiky, but for someone like me with a slow bow 36' is 20% increase over 30', it could make a bit of differance. The question that remains is how much arrow drop can be attributed to gravity versus the slowing effect of friction as the arrow cuts thru the atmosphere?