Jeff wrote in :

The bandwidth does not have to match exactly, but it it is too much then
it will distort as it exceeds the Bandwidth if the IF filters in the
receiver, and if it is too low the volume out of the receiver will be
low, so you would have to turn up the volume control when you used the
device. If it were extremely low then the audio would be very quiet
indeed, and if the signal strength were also too low you would loose
everything in noise.

The nominal max deviation in the FM broadcast band is +/-75kHz (150kHz
total).


Thanks. It looks like matching is best for compromise then, not some value
for transmission lower than reception, because what you gain in RF reach you
lose in SNR on output so striving for anything but a good match looks like a
game of diminishing returns.