I have read that shielding the cavities affects the tone, reducing the highs.

It depends how close the shielding is to the pickups,
and to a lesser extent, how close to the wiring. Directly adding
shielding to the pickups is the most noticeable. Adding shielding
foil/paint to the cavity has little effect on the sound. And using
PLASTIC covered wire instead of cloth could easily compensate the loss
of treble from the shielding paint.

Shielding can noticeably reduce noise and less noticeably reduce
highs. In my former life as a repairmen, I came to the impression that
players who are having a real 'problem' with single-coil noise will
likely be less than pleased with the noise-reduction, and some felt
the results weren't worth the cost.

So I usually try to downplay the noise-reduction effect, and also
remind folks about the perhaps noticeable loss of highs, but that's
mostly me being (overly?) cautious about customer's expectations.
Many high-end guitars, and even some Fender Teles, were fully
shielded. I guess Iwould recommend it for manufacturing guitars,
and caution that the cost and hassle of adding it to existing guitars
might not be enough of a difference to bring joy. But some guys are
very happy with the results. All of us are not in exactly the same
playing/noise situations, and I think that can also make a difference
in the outcome.