I might use Iridium on a big compression, forced induction, etc motor. They are neat since they conduct pretty well, but also last decently long (kinda like the best of plat and annealed-copper plugs). They also have a thin ground strap and electrode which is cool for shrouding concerns.
Warren, if you dont have some decent mods that can take advantage of these benefits, I dont think you would necessarily notice a difference.
Just MHO.