And so it continues.....custom stereo install

[fury fan]

Well, I will not argue with your real-world results and your satisfaction.
But - the flapper-doors at those vents is what stops the airflow - you have deleted that.
Those vents are fed by the cowl grilles/plenum, and the base of the windshield is a high-pressure area.
So your speaker is now effectively the seal to that pressure. So the faster you drive, the more pressure on the backside of the cone.
Which helps in 1 direction of cone movement, but resists in the other. Moreso than normal air cushion in a sealed enclosure, I would think.

Question - on your rear package shelf:
You have made some nicely-coved features in that panel, and your vinyl is nicely held to it.
I've always struggled with inside curves and vinyl.
Is that contact cement + vacuum bagging to get that to stick?
I generally use a thin layer of felt or foam to give some cushion, to protect against gouges and damage, an inside curves tend to bridge, rather than follow the contour.
What was your technique on that?

 

[turbinebronzebliss]

For the speakers, I don't notice any backpressure or change in sound at any speed. With 250 watts RMS powering them, it would be difficult for my trained ears to tell if there are any frequency losses or amplitude changes at all. It's really remarkable actually.

As for the package tray, I use wildwood which is a convertible top contact cement. It needs zero adhesion promotion and I prefer not to add any cushioning on my vinyl covered panels. This stuff will stay stuck for 10 plus years easily. I also use 4-way syretch vinyl always in my installs.