Steve, do you have any numbers or observations about which version decreases the road noise or the low frequency boominess more???
I don't have any actual numbers for the Model Y, but all of the owners that I am talking to are reporting that the car is quieter and less "Boomy." I have done specific instrumented tests on the Chevy Volt, with and without a VoltShelf. two customers repeated these tests at my direction with more sophisticated equipment. All of the results were very similar. With a VoltShelf in place, the cars were about 2 db quieter. This was measured at 45 MPH and 65 MPH. (70 db without a VoltShelf, 68 with one.) 2 db may or may not seem like a big difference, but you have to consider that sound is measured on a logarithmic scale not a linear one. Without getting into the math of it, the bottom line is that this is a reduction of about 20% in the interior noise level. It was clear that noise coming from the trunk area was greatly reduced.
Regarding the "Boominess" of the Model Y, I am not surprised by this at all. In my head, the great glass roof was always going to make the car be a great big echo chamber. No headliner up there to dampen out sound waves, they just bounce off and keep going. The rear hatch window is acts the same way. So with nothing to stop the sound coming from the rear, you've got a pretty loud car that by design will be a bit boomy. Give all of that, a hard parcel shelf like the ModelYShelf should easily absorb a lot of sound waves, stop them from coming up through the trunk area, and cut off part of the echo chamber. I would expect instrumented tests to yield similar results to the ones from the Volt above, and very possibly slightly better. I reserve the right to be wrong, but the anecdotal customer reports about this so far are headed in that direction.