[ChrisBennet]

While Saabs way of using the spark plug is ingenious and effective the method Porsche/Bosch uses work well enough. The 911 engine is not "too noisy" for the factory knock sensors so it must be possible. When I inquired about the J&S system they told me they were being used on a few 911s.

I looked into using the factory 964 sensors on my 3.2 (3.4 twinplug). the 964 motor has a "bridge" that mounts to bosses on the cylinders. These bosses aren't present in earlier motors so I didn't pursue it further.
-Chris