I remember a long set of discussions on the Porsche list on this very topic.
The bottom line was that for a vehicle whose ignition system was marginal
the splitfires would give a 'noticeable' improvement.  For a vehicle whose
ignition system was up to scratch, with good plugs, there was no
improvement.  The moral being go spend the money on real components, not a
gimmick to compensate for a tired ignition system.  Tim Smith