It does not help. However, it does appear from observation that _reducing_
the bandwidth to the IBOC analog requirement makes the majority of the
narrowband AM receivers out there sound better.

"Sound better" by whose judgement? That of iBiquity supporters?
People who leave their radio's treble knob turned all the way down
(and yes, I've encountered numerous people who do that)?

The engineers at 710 WOR discovered that a 6 kHz audio cut-off sounds
far better than 5 kHz, even on narrowband receivers -- and the
experience of myself and many other listeners supports that as well.
But, iBiquity won't let them use it, because it doesn't meet the IBOC
system spec.

But WOR does switch their audio to 6 kHz at sunset when the IBOC
sidebands are turned off, and on the average cheap radio, the
difference is subtle, but unmistakable. If you try to process an
Optimod 9200 too heavily when its 5 kHz filter is in use, you get a
very nasty-sounding "ringing" distortion. But when you switch the
9200 to its 6 kHz filter (which actually has its cut-off at about 6.25
kHz), this problem disappears and that extra kHz's worth of audio
response provides a significant improvement in crispness and clarity.

If AM stations really think that nobody is listening to them with
hi-fi receivers, then I would accept a 6 kHz cut-off. That's what AM
stations in the U.K. are using, and while it's definitely not hi-fi or
even mid-fi, it does sound alright. But not 5 kHz -- on an Optimod
9200, the distortion is intolerable, and on other processors, you're
still left with very dull-sounding audio.