As Hoyt as mentioned, the sound deadening material (I used Dynamat) and the fiber fill helps give the speaker a warmer more mellow tone. I feel it gives the speaker a little more punch. The one drawback I've noticed, is when it mellows out the speaker a little, I feel I lose a little volume. So in my opinion their is a little bit of a trade off. It's always good to experiment a little to see what works best with your system, this the reason mine has been apart a couple times this week.
I have plenty of volume with the Mmats in the fairing, plus they have help from the lowers with the DB G2'S, so I have gone back to fiber fill as well.
I've also been trying to make room in the fairing for some new gear, but should easily be able to fit everything in that I'm busing with my cables still across the top.
Going to try and squeeze a Dayton DSP, a Massive TX48, a Stinger, and a NVX Vadm 4 under the fairing. I've already done a dry test fit, so I'm sure I can make it work if I make myself a custom amp bracket.
As for the extra weight. I'm on an 05 ultra with the addition of speakers, lowers, and driving lights. It feels heavy in the parking lot, but rides like a Cadillac on the highway. 80 mph straight as an arrow with no hands for miles air drumming, lol. I'm a fairly aggressive rider scrapping floor boards in the corners and the extra weight doesn't phase me much other than when I'm backing up in a parking lot.
I rented a Street glide for 4 days of ripping through Australia a couple months ago and couldn't believe how light the front end felt, but once on the highway I don't really notice any weight issues.