I just wanted to echo Wankel7's post about the wheel wells...
As you may remember, I was extremely disappointed with my soundproofing results, after the doors, door cards, and the rest of the entire interior from front to trunk was done. I was still seeing 78-80 db at 70mph on a quiet highway.
I recently finished up a big job on the wheel wells. I used "Noico 80 mil 36 sqft car Sound deadening mat" on the liners:
https://www.amazon.com/gp/product/B00URUIKAK/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
And I sprayed the wells using 3M 03584 Professional Grade Rubberized Undercoating - 16 oz.
https://www.amazon.com/gp/product/B002H9CMCQ/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
The Noico stuff is flexible enough even in a cold garage to work with, and cuts easily enough to fit the weird shapes of the liners.
I cleaned and scrubbed the wells then taped things off before spraying a full can of the 3M stuff on. I didn't tape off the screw holes, and just was careful to avoid covering them too much. This worked out fine. I didn't just spray the wells in the front, but also the longitudinal braces in the frame - what I could get - as I figured those too transmit noise back into the cabin.
The big struggle was to get the liners back in, as the tolerances had changed with the Noico stuff on them, but I got there. I only couldn't get at one screw in the rear left well, as I needed another pair of hands. Otherwise, they went back in well enough.
The result? Using a new decibel meter (my old one was no longer supported) on an iphone, on the same stretch of highway, I got 71-74 decibels, while my buddy's 2016 GTI read 70-73. That's a six decibel difference, folks, or four times less in sound pressure. I couldn't believe it, so I checked it again several days later and got the same result.
The car still transmits high frequency noises into the cabin, no doubt through the window glass, but otherwise, I'm pretty damned happy and there's more to come.