Depends on what you want to do. On a stock setup - there's not much point. OEM platinums are actually pretty good and long lasting.
If you are setting your car up for more performance - and have done other mods - then going colder on the plugs will help supplement what else you have already done. Which specific plugs you should go to - however depends on how much modding you already have.
As you know going too cold for any particular setup is just going to waste gas.
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Regards,
Chew
2007 Sly Silver Pontiac Solstice GXP