They're only "performance" as far as how long they last. The purpose of Iridiums is to save the dealership money while your new car is under warranty. They never have to change them. But for people like us who change our plugs ourselves, they're a complete waste of money.
The gap has the most impact on performance, and you don't alter that from spec unless you've changed the compression or boosted the motor.
Really the only thing a spark plug can do is have a small ground strap that doesn't get in the way of the fuel mixture. You want the mixture as close to the spark as possible. That's why I laugh at those Bosch plugs that have multiple straps. First of all, electricity will only go to the strap with the least resistance. So basically, as I said earlier, the only performance here is longevity. But this slightly hurts your actual engine performance.