I've heard of electrodes breaking on spark plugs, and that is scary, but I don't see any reason to believe that a cold plug is more likely to have a breakage than a hot plug. I honestly believe a hot plug would be more likely to have a breakage.

Regarding your boost level, I strongly believe you should be running a colder plug even if you have no boost at all. I've run colder plugs on 2 lightely modded and tuned NA S2000's with great results. The stock heat range (7) is hotter than they need to be, and you are just askign for trouble running them in a modified / tuned S2000, especially on that is boosted.

 

After thinking about it some more, I really believe a hot plug is more likely to have an electrode breakage, since hot plugs have longer insulators.