My year engine calls for a AC43N plug. If I were to go colder in ratings, does this reduce the temp in my cylinders and therefore allow the car to run cooler? or am I missing the point. Also how does changing the ark plug affect performance?temp rating of a sp

The rating of a spark plug has nothing to do with the engine's operating temperature.  It's designed for the operating environment of the particular engine.  The temp of the spark plug's firing end has to be low enough to prevent pre-ignition and high enough to keep from fouling.  Too hot a rating can cause detonation and potential engine damage...too cold a plug and you can have deposit buildup leading to fouling and misfires. 

High performance, high compression engines tend to have colder plugs than the same engine with lower compression.  If you have a stock engine, keep the heat range that the manufacturer specifies.  If you have a built up engine, you'll need to get some advice from a specialist on proper heat range.  Forced induction engines tend to need spark plugs with recessed tips rather than extended tips. 

 

Another day, another lesson.

Bruce is right on the money. Colder plugs won't run better if it is designed for hotter plugs. Too hot a plug will lead to detonation and piston meltdown.
You might be able to run one step colder on your blown engine, depending on compression, the quality of fuel, and amount of boost you run.