Correct, the higher the number, the colder the plug. For FI applications, it depends upon the amount of boost you intend on running....I plan for between 15 to 30 lbs, so that meant 2 heat ranges colder, which is an 8. The stock plug is a 6 heat range. Lextreme's plug is 1 heat range colder, which may be perfect for your nitrous set up. Again, I am not sure about nitrous....Never researched it. All NGK plugs are made out of copper cores, and coated with other materials. A lot of the plugs used are copper/nickel for FI set ups. I have heard some state that platinum or Iridium are not good for FI, but I have not seen any evidence to back this up. It seems that the heat range is critical though. The colder the plug, the more the chance of fouling the plugs, so you want to match it up to your set up.
Hey plug and FI experts please chime in....
Ryan