I do road racing with another car of mine, and with my engine I've tried a bunch of things, and what I've found out and what others have too, is that the most important thing is the temperature range of the plug, and the gap. The other fancy plugs cost a whole lot, but don't do much of anything with regards to power gains. Having the right type of plug with the right gap makes the biggest difference.
The platinum plugs last a lot longer, and that's about all that makes them better as far as what my friends and I have discovered in tuning our engines. I agree that the fancy plugs are a gimmick to sell plugs for more!
Now, this info I've found out with regards to my 911 engines, but I think it holds true for my Navajo as well. Look at all of the big racing groups and their cars - they've got platinum plugs - not the fancy ones. That's gotta' mean something!
I hope this helps a bit. It's just my insight on the plug subject from what I've noticed.