He doesn't need to demonstrate the impracticality in that. Everyone already knows that measuring effort is impossible in practice.

edit: His final claim -- that the tax on height is an explicit consequence of a utilitarian approach -- is simply flawed (or irrelevant, actually) because we do have a tax on height. A progressive income tax is meant to approximate what would happen if we accounted for all of these variables, but without actually accounting for each one explicitly, as that would be impractical.

Now, I'm not saying that the utilitarian method is the end-all. If it is flawed (which I think it is), this is simply not the flaw.