The performance gain from double plats is probably close to nil, but most likely there. For me? I have new double plats, and I'll probably never change plugs again. If you're going to get new plugs because you need them, the $10 or so extra might net you a couple hp, depending on your setup. Before it was a choice between long-lasting double plats and a little bit more fire from fine-wires. I chose the ones I won't have to touch, others (Demon is the only one I know of) run fine-wires and change them more often. I would assume that less-stock engines will get more from the better spark, but I don't really have any tech on that, it's just a trend I've seen.
