Precious metal plugs like these are called performance for their resistance to electrode erosion. They maintain gap for a longer duration. Generally, they do not outperform standard copper electrode plugs (by outperform, I mean produce a stonger spark, which is the essential purpose of the plug in the first place). All other things being equal, a smaller center electrode (compare a platinum or iridium plug to a standard plug and you will see the electorde in the former two is much much smaller than the one in the standard plug) will produce a weaker spark. All other things being equal, and assuming the ignition system can deliver voltage sufficient for the purpose, the larger the gap between the center electrode and the ground, the stronger the spark produced. The reason is that the potential generated at the electrode tip must be much higher at ionization to cause the charge to cross the larger gap. Unfortunately, our DI cassettes, while capable of delivering a very potent voltage sufficient to run larger gaps, also suffer from suspect endurance. They are failure prone parts, and expensive, so it is highly recommended to keep the plug within the specified gap range. Too large a gap makes the cassette work harder, shortening its already too short life. For this reason, it is a good idea to use erosion resistant plugs or to closely monitor standard plugs. I personally prefer the superior conductivity and stronger spark of a copper based electrode and change the plugs every year, checking their gaps and cleaning at least once between changes.
The Viggen does come with platinum plugs, but (in the experience of my aero) they don't run as nice or as smoothly as copper plugs. I have the BCPR7ES-11 gapped to .95 mm and I'll never use those platinums again. You can feel the car just likes copper plugs more, best to give it what it wants. If you want better performance, get the colder 7's.
Actually all 9-3s, not just the Viggen, had platinum plugs put in at the factory at a certain point. The feelings over on that other bulletin board are that they work just fine, but they need to be regapped, although not replaced, at the same interval that the copper plugs need to be replaced, and the increased life is about the same as the increased cost.