Having messed with multiple configurations on the Rushmore audio system on my 2015..................................
Regardless of speaker / amp configuration the addition of poly fill did not noticeably increase my "road volume". However the addition of poly fill did result in a slightly detectable tightening of the perceived bass response. I've used loose poly, as well as poly stuffed inside of panty hose and then also an experiment with the poly stuffed inside of large freezer bags and placed down inside the pods to where they barely cleared the backside of my speaker magnets. In fairness I've never gone through the extensive prep work as OP #2 did.
Regardless of my method of putting poly into the pods, there wasn't any noticeable difference in SQ or volume at road speeds (other than the slightly noticeable improvement in bass response and tightness of the bass).
I finally settled on loose poly + sealing the pods after splitting them open and reassembling them. I used off the shelve silicon for that project.
In all honesty, if you plan on keeping the stock speakers, and stock head unit without upgrading to better speakers and an amplifier you're pretty much pissing in the wind relative to achieving your stated goal.
Good luck whatever route you go though.
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