[rotrex]

I use a J&S, too. 20 deg is too harsh unless you have a very slow burning combustion chamber design, e.g 60ies to 70ies Hemi heads etc. 10 deg worked best.
With the J&S serious knock is well captured. You can still hear it and also feel it from the power loss of the pulled timing under boost. I usually use it to point me to the map spots that need some correction. I had a pump issue once with the priming pump controller of my Aquamist 2c not working properly anymore and the J&S had some serious work to do as WI essentially failed. The engine spit blue flames from the exhaust with serious misfires as the J&S pulled 10 deg on all cylinders. That was a nice light show on the gauge. But nothing else happend.
As I am converting to an other engine I have sold my current engine and J&S last week.
The J&S is not as smooth as a fully tuned factory knock control system, but it does its job to indicate knock and provide some monentary correction. For street tuning, it is very very nice to have.
I would not "run" on it as in dail in full timing and the J&S does the rest.
I still can recommend it. It is 1000% better than having no means of interventing on highly strung forced induction engines.