My two cents: I believe we need to consider the change in magnitude of
the mass of the key, which would affect inertia.

Adding six inches to the overall length of a key is probably not enough
of a change in the magnitude of the mass of the key, that the change
in inertia could be detected by most pianists.  From my limited
experience, it seems that keys are fairly light weight anyway and
adding such a small amount of extra material should not effect the
overall magnitude of the mass, therefore inertia, of the key to the
point that it would be noticeable.

Tom Lear
San Francisco, CA