I think you're talking about the "float"- which controls the fill-valve. The basic problem with changing the amount of water is that the whole "flush-physics" is designed to work with a optimum amount of water and either more or less messing up the hydrodynamics so the flush doesn't work effectively. I have even heard that even mineral deposits that builds up on the trap interior over time (and which can not be cleaned by conventional means) can change the flush-physics enough to make the flushing less effective. So its not a simple matter to engineer a toilet to handle two entirely different kind of waste efficiently.
(note how delicately I'm handling this discussion- keeping it on a high plane of G-rated decorum)