The trichord dampers do not extend much beyond the mid-tenor because
the tips of the felt drag along the wires, and on the shorter wires
they have a greater affect. Also some harmonic over-ring is wanted
for tonal color.
Where the dampers end is a matter of sound board impedance: usually
around note 67 is the stopping point. In an area where the strings do
not have as much power and sustain, their tone would be lost if they
were dampened. Some pianos require dampers progressing higher into the
treble because the sound board is more efficient.
If you would like more info regarding sound board impedance, look in
the archives of the pianotech forum at <ptg.org>: sound board crown.
There is more there than you probably care to know.
Jon Page, piano technician
Harwich Port, Cape Cod, Mass.