Without making too much out of the loss of treble / sparkle due to ground/shield capacitance, it is true that this effect is quite measurable and audible, and can also lead to undesirable side-effects.
The Zexcoil fellow recently studied shielding on cabling from pickup to
controls as a result of some anomalies noticed in cable brands, and as a
side-issue he measured also the capacitance effects of cavity
shielding. I believe he measured conductive paint (graphite) used to shield pickup cavities, but one should expect that even more dramatic effects will be found when using copper shielding.
Some recommend copper as preferable because it is a much better
conductor, and a better Radio Frequency (RFI) shield for noise
reduction. And this has become more important in a heavily polluted
radio environment we now live in, with multiple sources of interference,
from cell-phones to satellites.
To get equivalent shielding with conductive (graphite) paint, one must
usually use two or three coatings and build a thick layer.
However, a disadvantage of copper is that it is also a natural capacitor.
This capacitance effectively bleeds treble to ground, and can also cause or alter natural resonances and frequency response.
So people have discovered on their own that removing either paint or
copper shielding restores the sparkle or clarity and dynamics of single
coil pickups, and even modern noiseless ones.
The drawback is that of course the noise and hum returns to the signal path.
I recently acquired a MIJ body already fully shielded with copper, and
routed only for single coil size pickups. Obviously the concern was
that I would lose the expected sparkle and dynamic response from either
single-coils or stacked noiseless pickups.
Looking at the problem closer, it was obvious that far too much
shielding was used to shield the cavity, and that much of it was
superfluous and could be removed while retaining most of the effective
shielding of the cavity.
For instance, as I will show in the photo, a large amount of the
shielding inside each pickup cavity can be removed without degrading
The inside facing, and sandwiched walls do not need to be shielded in
these cavities, because they are already shielded by the outside walls
Removing the copper from these sides dramatically reduces the stray
capacitance of the cavity, while preserving the shielding against
outside RF sources.
Here you can see where I have cut out the top half of the shielding in the middle pickup cavity. The middle pickup is certainly heavily shielded already from being sandwiched between the other two cavities. Stray RF coming at the guitar endwise is caught by the outer shielding and the other pickups.
Here is shown that much copper laying in the bottom of the cavity has been cut out, drastically reducing the stray capacitance expected along the cables/wire leads running from pickups into the controls area. This copper does little to shield the pickup network from RF, but adds a significant amount of capacitance to ground for the signal path. Also, the copper is an added hazard that can short out or ground bare wire, eyelets or solder-points on the back of a pickup, if it is lowered too far into the cavity.
If the pickups are not suffiently brought back to life by this removal,
one can go even further, either leaving narrower strips, or simply
disconnecting most of the copper from ground. There is no need to
remove all the copper unless the results are not satisfactory. Its a
law of diminishing returns here, and some shielding is desirable, even
with "noiseless" pickups.