Just stick with the OEM recommended plugs. I beleive that the temperature of irridium and platinum plugs in the same car may not be the same.

My motorcycle had irridium plugs that lasted almost 30,000 miles and were still working fine when I changed them. 30,000 in a motorcycle is probably comparable to 120,000 on a car since the RPM's average 4 times higher. Most newer high performance motortcycles with fuel injection all use irridum plugs.