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10 decibels is a 10x reduction in energy. An 8dB reduction is a factor of 10^(8/10) = 6.3 times less energy radiated. Good luck getting a 6.3x improvement in any other parameter in an aircraft design without major sacrifices elsewhere.

At what distance? Sound takes quite a bit of power to generate. It's likely this saves a noticeable amount of fuel (maybe over multiple flights). If it's an attachment this may not apply.

After a certain point changes are incremental. We're at that stage with jet engines.

A 10 dB reduction is half the perceived loudness, so 8 dB is quite significant. 142 dB (SPL) will make you deaf instantly if you are right next to the sound source, but at a distance the 8dB reduction will make it sound much softer.