There have been a couple guys running 350 rwhp engines that have gotten buy just fine with the stock double platinum autolites. I can recall XR7 Dave posting that he had been running the standard autolites for quite a while. If I recall one of his posts it wasn't until his latest tuning round on his autorotor car where he was seeing spark blowout as RPM's increased and boost pressure was hitting over 21psi. Maybe he can comment more on this if he gets a chance.
For the cost, the stock double platinum autolites are just a killer spark plug that seems to work really..really well in our cars. Our ignition system is a bit of a twitch unit. We're getting good spark, good combustion at 12psi -15psi, where folks with vortechs and such running anything over 6psi get spark blowout on their 4.6 motors unless they run an iridium plug. I don't know that anyone has figured out why this is.
There certainly isn't any harm in using the irridium plugs. The only down side that I've seen is that since our system is a wasted spark system. The plug will see energy flow both directions through it. That's why stock is a double platinum tipped plug, so that both the tip and the ground electrode don't wear away. The Irridium plugs I've seen from anyone but the new AutoLite irridium, don't use irridium or platinum on the ground conductor. Thus in our ignition system you might see some excessive wear on the ground conductor with a standard irridium plug.