I do not have any documented evidence but I would agree with all that you have stated. With the visor down the sound protection is increase (from what I have witnessed).
I have been wearing one (with anr) for the past 10 years and would do everything to protect my hearing.
They also protect your noggin in the case of;
- Head Impact
- Bird strike (through the windsheild) whilst visor is down
- partial protection from FOD when outside the aircraft with visor down
- Would you ride a motorcycle without a full face helmet these days?
Cheers