An interesting topic on The Firearm Blog.

It is good that the author also lists muzzle energy, not just velocity.

The Hornady load yields 1176 J in the test, compared to 1107 J of the Speer Gold Dot. That is 6 percent more. On the other hand, it is still about 30 percent less than an ordinary load from a 20" barrel delivers.   

The author observed reduced flash of the Hornady load, but interestingly does not mention the claimed reduced muzzle report.  

Muzzle report is extremely subjective even if you're set up to measure decibel levels since tiny changes in placement positioning and etc can easily swamp the improvements an ammunition load could possibly give pretty easily

Isn't the difference between muzzle and ear measurements usually 10 decibels or so?

I do not know...

The one thing I know about this particular subject is that suppressor makers and etc have long since figured out how to game "the measurements" to a point where nato is developing a hard spec for how to do the testing because it's quite possible to game things heavily enough to make things look good that aren't.

I highly suspect that this quiet ammunition claim would be very subject to getting lost in the statistical noise.

It's sorta like how If you're doing precision shooting at 1km or less you don't calculate in coriolis because the amount of deviation it will induce in POI is smaller than the potential errors from other things by so much that it's effect is swallowed up in the noise from other factors.