You must have had some pretty cruddy plugs beforehand. Platinum plugs won't give you gains that you can feel like that. The good thing about Platinum plugs are that they last forever, same with the iridiums (that's why all the V6 people love em). For four cylinder people the joe shmoe NGK plugs are fine (they are what come stock) you just gotta replace em once a year or every 24,000 miles, whichever comes first (or around that, don't quote me, look in a manual). As for you question though, don't waste money on that kind of crud, unless it needs replacing, in which case I'd just go with stock. Save your money for good stuff like a CAI, header, exhaust, that type of thing.