My experience with iridium plugs is that they do improve cold starting a bit, and cold running, as well, when "Cold" is below 20F. I believe this to be a product of lower voltage requirement, resulting in a better spark at very low temperatures. Besides that, they do not deteriorate for a very long change period.
Before I started using them in my Beemer motorcycle, it took a fair amount of cranking, and some increased throttle above and beyond the fast idle ramp. Now, I can actually start it without the fast idle, adding the fast idle after it lights off.....At least to the limit of temperatures I am willing to ride it in. (20F is pretty much my lower limit).