That's a good question - when i was working over the carb on the '95 i had several new cheap champions to get a read on the plug and when it was about right then pop'd in the iridium ngk dpr8eix-9, that was about 8k miles ago, cold and hot start ups are fast - seems like the engine turns over about twice at the most. so longivity maybe my answer.  

Fouled the plug after about 3000 k. on mine, bought a champion and it's been great ever since...choke it and she fires right up after a couple of turnovers so i figure you can't get much better than that...

With the Iridium, my bike started slightly faster, but after two months it starts slower than ever. I don't notice any appreciable improvement of the power delivery.