I don't think there is an easy answer or one for all occasions or rivers.
There is no doubt that the sound of bait / feeder etc. hitting the surface/ bottom on heavily barbel populated waters, where the fish have to compete, often brings them up to the spot very quickly.
Likewise, it generally, would not work on the Ouse very well, when the expectation is one fish every few trips and many anglers will rove to find a feeding one or leave a bait out possibly all session.
As Chris mentioned, I have the opportunity to watch fish frequently and they will often respond immediately to the sound of bait hitting the water. I have even caught some fish before the feeder has hit the bottom in 5 feet of water. I have also spent many hours watching reactions to line in the water.
My opinion, for what it's worth is that I prefer a line that fish can see, and expect, rather than an invisible one that shocks them when/if the bump into it.
One of my friends uses Stonze and is very succesful on a very tough river. But if we are debating the difference between the noise a Stonze makes against a lead as it hits bottom as being important..................We really need to get a grip.
The majority of my customers over elaborate on their rigs, and frequently change to the "New" and latest gadget/edge but lose sight of its potential disadvantages whilst swallowing the tall stories about it's effectiveness.
After reading a recent Paper report about a fish caught on a fantastically trusty (insert name) rod it transpires it was not that make at all. This is where the Writer bends the truth and should be ashamed of himself. It happens all the time.
One of the biggest advantages of stonze in my opinion is that they are simply heavier than normal leads used and give more of a bolt rig effect.
Many (note . I said, many)of the advantages attributed to back leads are as a result of the same reason.
There is no doubt that somedays the fish appear more cautious than normal, and no doubt some form of association with uncomfort exists.
Then again, having caught the same barbel in a session twice on more than one occasion, It does make you wonder.
As an experiment I once caught the same salmon parr 11 times in 30 minutes.
I reckon I could catch more 5lb chub on a large piece of flake and a mashed bread feeder with 10lb line and a size 4 hook than with a size 18 to 2lb bottom....and as our old angling friend once said (or similar)............... The chevin is a most cautious of fish.
Graham