I seem to remember this one doing the rounds before.
(I think I may have asked it). The answer then was more or less this.
The blades have to lift their own weight as well as the rest of the machine. Any weight you add comes off the payload unless you get an increase in maximum gross weight certified. (which you almost certainly wont). The weight and balance schedule will need to be amended.
The same applies if you add avionics, soundproofing, fluffy dice etc.
Phil