From what I have read about ST13s using the same Iridium plug as the ST11, there have been no issues with the plugs. The Iridiums seem to be able to cover the heat ranges of the two standard plugs, and this was for riders who were able to ride at 130kph plus in summer in the US. The heat rating is more to do with running the spark plug at the right temperature for keeping the plug clean. If we were able to ride flat out for hours on end then a cooler rated plug might be advisable, but it may just foul up when you ride at normal speeds. A hotter or colder plug does not affect the running temperature of the motor.
I had my first set of Iridiums in for 70,000kms, and only replaced them because one plug fouled one day due to the bike running too rich on that cylinder over a long period of time (I have since replaced the offending carb parts) The standard plugs can last ages longer than recommended.
Years ago in my Suzuki GS1000 after I had it dynoed to check that some K&N filter pods were tuned properly, they put in one stage colder plugs. In only a few 100kms the plugs were fouling up so back in went the standard plugs.