Colder heat range plugs for our car (note!) are a bandaid solution for too advanced timing. Since the turbo is on boost or running full advance timing (the two situations when cold plugs are often used) only a very small fraction of the time, the cold plug is wasting fuel/power all the rest of the time.

What this means is that the timing is way too advanced or the boost is set too high for the rest of the engine management. It is just a trick to keep turbo cars from smashing pistons when the development work hasn't been done. Same deal with coolant thermostats (the plate, not the sensor).

Cold range plugs are a useful trick in NA cars designed to run higher octane, where the engine management is really crude to begin with. In turbo cars the (some combo): intercooler, octane, radiator, timing, and static compression should be modified to use the boost effectively.