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Looks like you're pretty conservative and safe there. I would say (but please don't just take my word) that you should keep the same plugs that came with the car.
I have heard from many people that the Zex plugs are absolute garbage and that the Iridiums corode with nitrous use. Copper is the way to go. I use NGK one step colder on mine. No problems yet.
Get the regular NGK's (just one step colder). No need to go buy $10 spark plugs when you are going to have to buy new ones in 6 months. They burn up pretty quick ...even with only a 50 shot.