Fido,
I think the claims that most people make about XX brand plugs making significant improvements in performance are partly psychological and partly due to the simple fact that the new plug is replacing a previous one that is fouled, mis-gapped, the wrong heat range or otherwise just not working properly. I've yet to find an unbiased report based on dyno runs or other tangible data that proves different. That said, some designs like the iridium and platinum plugs reportedly resist fouling better than others, and/or maintain their optimal performance over longer periods of time, meaning they need to be replaced less often.
I just replaced my factory spec plugs with iridiums and did notice it ran a tad bit smoother and had a little better response, but I'd also say the plugs I swapped out were overdue and the percieved gains were not likely due to the mere fact that I installed the iridiums.
