Is there actually any scientific info on the effect of underboring?
If by underboring, you mean an interference fit between the ball and barrel, would that not require more force to push the ball out at the same fps? Also, paint is not uniform - as soon as you get that ball that's just a tad larger than the others in an underbored setup, you get a ball break.
I get the bro science behind this - tighter ball, less air can escape - sure, I guess. It's not substantiated.
Something loud rapidly expands, something quiet slowly expands.
A marker with a large amount of volume of air per shot can use lower pressure air - PV=nRT, which basically translates to PV=energy.
Now, it takes the same amount of energy to fire a paintball at 300fps.
So, we have two ways of achieving the same energy - Pressure*Volume = constant, so we can either have high pressure, low volume, or low pressure, high volume. In general, a poppet works the first way, a spool the second way. Of course, there is everything in between.
And we arrive at the Etha. The Etha is in between. It will never be that quiet because of the inherent design of the marker - there is only so much volume that the air used to fire a paintball can inhabit without having a wildly inefficient and inconsistent shot.
Things like porting and barrel modifications and bore to paint size are small things that may or may not have a noticeable effect on the sound level. The primary thing though, is what I explained.
Furthermore, due to the design of the Etha, dwell tuning will not work very well - dwell tuning works best with markers that have a large, preferably controlled volumizing chamber to work with, like most true spools.
Tl,dr; your Etha is loud because it's an Etha. You can try things, but short of making a silencer shroud, your only going to hurt performance pretty badly with marginal gains in sound signature.