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Your best bet would be to follow pingu's diy. It is a lot like the PLP intake, or the Joez intake for the IS300s, 2IS', ISF's, etc. Then just do a high flowing drop in filter of your choice and you should be good to go.
This method is best I think because it retains the factory air box and you don't have to worry about the MAF sensor. Good luck .
EDIT: I just realized this question was about an IS250, not a IS300 like I originally thought.
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