From what I have read, here and on GTA, tuning for efficiency not only makes a noticeable difference in Db at the muzzle, but also in accuracy.
If I understand it correctly, an airgun tuned to maximum efficiency uses just as much air as needed, so very little is wasted once the projectile leaves the barrel and therefor doesn't affect the accuracy negatively with a blast of excess air behind the projectile once it leaves the muzzle. I don't know if tuning to the "knee" is the best way to achieve this, but...
Again, from what I have read, you may get the best results in lower noise and accuracy by using the highest PSI and shortest dwell of the HS that will make the projectile leave the muzzle at the desired fps. Finding the perfect mix of those 2 is the key.
Cue those folks who have actually done testing in this area to chime in and either confirm/deny or amend what I just posted. (grin)
P.S.
If I understand what I said above correctly, then tuning for efficiency works regardless of barrel length for the most part.